Create CONTRIBUTING.md

CONTRIBUTING.md

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+## How to contribute to yavdb
+
+### Main rules
+
+* Before you open a ticket or send a pull request, [search](https://github.com/rtfpessoa/yavdb/issues) for previous discussions about the same feature or issue. Add to the earlier ticket if you find one.
+
+* If you're proposing a new feature, make sure you create an issue to let other contributors know what you are working on.
+
+* Before sending a pull request make sure your code is tested.
+
+* Before sending a pull request for a feature, be sure to run tests.
+
+* Use the same coding style as the rest of the codebase.
+
+* Use `git rebase` (not `git merge`) to sync your work from time to time with the master branch.
+
+* After creating your pull request make sure the build is passing on [CircleCI](https://circleci.com/gh/rtfpessoa/yavdb)
+and that [Codacy](https://www.codacy.com/app/rtfpessoa/yavdb) is also confident in the code quality.
+
+### Commit Style
+
+Writing good commit logs is important. A commit log should describe what changed and why.
+Follow these guidelines when writing one:
+
+1. The first line should be 50 characters or less and contain a short
+   description of the change prefixed with the name of the changed
+   subsystem (e.g. "net: add localAddress and localPort to Socket").
+2. Keep the second line blank.
+3. Wrap all other lines at 72 columns.
+
+A good commit log can look something like this:
+
+```
+subsystem: explaining the commit in one line
+
+Body of commit message is a few lines of text, explaining things
+in more detail, possibly giving some background about the issue
+being fixed, etc. etc.
+
+The body of the commit message can be several paragraphs, and
+please do proper word-wrap and keep columns shorter than about
+72 characters or so. That way `git log` will show things
+nicely even when it is indented.
+```
+
+### Developer's Certificate of Origin 1.0
+
+By making a contribution to this project, I certify that:
+
+* (a) The contribution was created in whole or in part by me and I
+  have the right to submit it under the open source license indicated
+  in the file; or
+* (b) The contribution is based upon previous work that, to the best
+  of my knowledge, is covered under an appropriate open source license
+  and I have the right under that license to submit that work with
+  modifications, whether created in whole or in part by me, under the
+  same open source license (unless I am permitted to submit under a
+  different license), as indicated in the file; or
+* (c) The contribution was provided directly to me by some other
+  person who certified (a), (b) or (c) and I have not modified it.