docs: add contributing guidelines

Signed-off-by: Alfredo Deza <adeza@anchore.com>

CONTRIBUTING.md

@@ -0,0 +1,103 @@
+# Contributing to `syft`
+
+If you are looking to contribute to this project and want to open a Github pull request ("PR"), there are a few guidelines of what we are looking for in patches. Make sure you go through this document and ensure that your code proposal is aligned.
+
+## Sign your work
+
+The `sign-off` is an added line at the end of the explanation for the commit, certifying that you wrote it or otherwise have the right to submit it as an open-source patch. By submitting a contribution, you agree to be bound by the terms of the DCO Version 1.1 and Apache License Version 2.0.
+
+Signing a commit certifies the below Developer's Certificate of Origin (DCO):
+
+```text
+Developer's Certificate of Origin 1.1
+
+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.
+
+   (d) I understand and agree that this project and the contribution
+       are public and that a record of the contribution (including all
+       personal information I submit with it, including my sign-off) is
+       maintained indefinitely and may be redistributed consistent with
+       this project or the open source license(s) involved.
+```
+
+All contributions to this project are licensed under the [Apache License Version 2.0, January 2004](http://www.apache.org/licenses/).
+
+When committing your change, you can add the required line manually so that it looks like this:
+
+```text
+Signed-off-by: John Doe <john.doe@example.com>
+```
+
+Alternatively, configure your Git client with your name and email to use the `-s` flag when creating a commit:
+
+```text
+$ git config --global user.name "John Doe"
+$ git config --global user.email "john.doe@example.com"
+```
+
+Creating a signed commit is then possible with `-s`:
+
+```text
+$ git commit -s -m "this is a commit message"
+```
+
+To double check if the commit was signed, look at the log output:
+
+```text
+$ git log -1
+commit 37ceh170e4hb283bb73d958f2036ee5k07e7fde7 (HEAD -> issue-35, origin/main, main)
+Author: John Doe <john.doe@example.com>
+Date:   Mon Aug 1 11:27:13 2020 -0400
+
+    this is a commit message
+
+    Signed-off-by: John Doe <john.doe@example.com>
+```
+
+
+[//]: # (TODO: Commit guidelines, granular commits)
+
+
+[//]: # (TODO: Commit guidelines, descriptive messages)
+
+
+[//]: # (TODO: Commit guidelines, commit title, extra body description)
+
+
+[//]: # (TODO: PR title and description)
+
+
+## Test your changes
+
+This project has a `Makefile` which includes many helpers running both unit and integration tests. Although PRs will have automatic checks for these, it is useful to run them locally, ensuring they pass before submitting changes. Ensure you've bootstrapped once before running tests:
+
+```text
+$ make bootstrap
+```
+
+You only need to bootstrap once. After the bootstrap process, you can run the tests as many times as needed:
+
+```text
+$ make unit
+$ make integration
+```
+
+## Document your changes
+
+When proposed changes are modifying user-facing functionality or output, it is expected the PR will include updates to the documentation as well.