Adds CONTRIBUTING.md.

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+## About contributing
+
+There are multiple ways you can contribute to this project.
+
+The easiest way to contribute is to report possible bugs, request features, [discuss ideas](mailto:krunoslav.zaher@gmail.com?subject=[RxSwift] I have an idea) and share excitement about this project.
+
+You can also make pull requests.
+
+There are some best practices that will be followed during the development of this project for common good ([Gitflow](http://nvie.com/posts/a-successful-git-branching-model/) branching model)
+
+So what does this mean in practice:
+
+* If you notice a bug in **documentation** only, that could be considered non risky hotfix, so please make a PR to **master** branch
+* If you notice a bug in **source code** please make a PR to **develop** branch because otherwise it could get a lot worse :) If needed, hotfix will be created from commit with the fix and applied to master branch after the PR has been merged into develop and tested.
+* If you want to make a small contribution (dozen lines of code) that is not a bug fix please make a pull request to **develop** branch.
+* If you want to make a big contribution to the project, please [discuss it](mailto:krunoslav.zaher@gmail.com?subject=[RxSwift] I have an idea) first with me so we can make sure project is going in the right direction. All pull requests with **source code** contributions should be targeted to **develop** branch.
+
+## 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.
+
+*Wording of statement copied from [elinux.org](http://elinux.org/Developer_Certificate_Of_Origin)*