Add contributing guidelines

as suggested at https://github.com/blog/1184-contributing-guidelines

CONTRIBUTING.md

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+# Contributing to GNSS-SDR
+
+:+1::tada: Thanks for taking the time to contribute! :tada::+1:
+
+Third-party contributions are essential for keeping GNSS-SDR
+continuously improving. We simply cannot access the huge number of
+platforms and myriad configurations for running GNSS-SDR. We want to
+keep it as easy as possible to contribute changes that get things
+working in your environment. There are a few guidelines that we need
+contributors to follow so that we can have a chance of keeping on top of
+things.
+
+The following is a set of guidelines for contributing to GNSS-SDR, which
+is hosted in the [GNSS-SDR Organization](https://github.com/gnss-sdr) on
+GitHub. These are just guidelines, not rules. Use your best judgment,
+and feel free to propose changes to this document in a [pull
+request](#how-to-submit-a-pull-request).
+
+## Code of Conduct
+
+This project adheres to the Contributor Covenant [code of
+conduct](CODE_OF_CONDUCT.md). By participating, you are expected to
+uphold this code. Please report unacceptable behavior.
+
+## Reporting an issue
+
+Have you found a bug in the code which is not in the [list of known
+bugs](https://github.com/gnss-sdr/gnss-sdr/issues)? Do you have a
+suggestion for improvement? Then by all means please [submit a new
+issue](https://github.com/gnss-sdr/gnss-sdr/issues/new), and do not
+hesitate to comment on existing [open
+issues](https://github.com/gnss-sdr/gnss-sdr/issues).
+
+When filling a new issue, please remember to:
+
+ * **Use a clear and descriptive title** for the issue to identify the
+problem.
+
+ * **Describe the exact steps which reproduce the problem** in as many
+details as possible. For example, start by describing your computing
+platform (Operating System and version, how did you installed GNSS-SDR
+and its dependencies, what file or front-end are you using as a signal
+source, etc.). You can also include the configuration file you are
+using, or a dump of the terminal output you are getting. The more
+information you provide, the more chances to get useful answers.
+
+ * **Please be patient**. This organization is run on a volunteer basis,
+so it can take some time to the Developer Team to reach your issue.
+They will do their best to fix it as soon as possible.
+
+ * If you opened an issue that is now solved, it is a good practice to
+**close it**.
+
+The list of [open issues](https://github.com/gnss-sdr/gnss-sdr/issues)
+can be a good starting point and a source of ideas if you are looking to
+contribute to the source code.
+
+
+## Contributing to the source code
+
+### Preliminaries
+
+   1. If you still have not done so, [create your personal account on
+GitHub](https://github.com/join).
+
+   2. [Fork GNSS-SDR from
+GitHub](https://github.com/gnss-sdr/gnss-sdr/fork). This will copy the
+whole gnss-sdr repository to your personal account.
+
+   3. Then, go to your favourite working folder in your computer and
+clone your forked repository by typing (replacing ```YOUR_USERNAME``` by
+the actual username of your GitHub account):
+
+          $ git clone https://github.com/YOUR_USERNAME/gnss-sdr
+
+   4. Your forked repository https://github.com/YOUR_USERNAME/gnss-sdr
+will receive the default name of `origin`. You can also add the original
+gnss-sdr repository, which is usually called `upstream`:
+
+          $ cd gnss-sdr
+          $ git remote add upstream https://github.com/gnss-sdr/gnss-sdr.git
+
+To verify the new upstream repository you have specified for your fork,
+type `git remote -v`. You should see the URL for your fork as `origin`,
+and the URL for the original repository as `upstream`:
+
+```
+$ git remote -v
+origin    https://github.com/YOUR_USERNAME/gnss-sdr.git (fetch)
+origin    https://github.com/YOUR_USERNAME/gnss-sdr.git (push)
+upstream  https://github.com/gnss-sdr/gnss-sdr.git (fetch)
+upstream  https://github.com/gnss-sdr/gnss-sdr.git (push)
+```
+
+### Start working on your contribution
+
+Checkout the `next` branch of the git repository in order to get
+synchronized with the latest development code:
+
+```
+$ git checkout next
+$ git pull upstream next
+```
+
+When start working in a new improvement, please **always** branch off
+from `next`. Open a new branch and start working on it:
+
+```
+$ git checkout -b my_feature
+```
+
+Now you can do changes, add files, do commits (please take a look at
+[how to write good commit
+messages](https://chris.beams.io/posts/git-commit/)!) and push them to
+your repository:
+
+```
+$ git push origin my_feature
+```
+
+If there have been new pushes to the `next` branch of the `upstream`
+repository since the last time you pulled from it, you might want to put
+your commits on top of them (this is mandatory for pull requests):
+
+```
+$ git pull --rebase upstream next
+```
+
+### How to submit a pull request
+
+When the contribution is ready, you can [submit a pull
+request](https://github.com/gnss-sdr/gnss-sdr/compare/). Head to your
+GitHub repository, switch to your `my_feature` branch, and click the
+_**Pull Request**_ button, which will do all the work for you. Code
+comparison must be always to the `next` branch.
+
+Once a pull request is sent, the Developer Team can review the set of
+changes, discuss potential modifications, and even push follow-up
+commits if necessary.
+
+Some things that will increase the chance that your pull request is
+accepted:
+
+ * Avoid platform-dependent code. If your code require external
+ dependencies, they must be available as packages in [Debian OldStable](https://wiki.debian.org/DebianOldStable).
+ * Write tests.
+ * Follow our [coding style guide](http://gnss-sdr.org/coding-style/).
+ * Write a descriptive and detailed summary. Please consider that
+reviewing pull requests is hard, so include as much information as
+possible to make your pull request's intent clear.
+
+For more details about Git usage, please check out [our
+tutorial](http://gnss-sdr.org/docs/tutorials/using-git/).
+
+
+## Contributing to the website
+
+The content of http://gnss-sdr.org lives in a GitHub repository at
+https://github.com/gnss-sdr/geniuss-place
+
+You can fork that repository, reproduce the entire website on your
+computer using [Jekyll](https://jekyllrb.com/), do changes and submit
+pull requests, just as explained above. For more details, please check
+out [how to contribute](http://gnss-sdr.org/contribute/).
+
+Last but not the least, you can leave your comments on the website.
+
+
+------
+
+
+
+![GeNiuSS
+contributes](http://gnss-sdr.org/assets/images/geniuss-contribute.png)
+
+Thanks for your contribution to GNSS-SDR!