Add contributing guidelines (#1135)

Add some basic contributing guidelines based (almost cloned) on the
Redis ones.

There were some questions about how to contribute and get started with
the project. I see this type of document in other projects that I think
can be useful.

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+Note: by contributing code to the Coturn project in any form, including sending
+a pull request via Github, a code fragment or patch via private email or
+public discussion groups, you agree to release your code under the terms
+of the BSD license that you can find in the LICENSE file included in the Coturn
+source distribution.
+
+# IMPORTANT: HOW TO USE COTURN GITHUB ISSUES
+
+Github issues SHOULD ONLY BE USED to report bugs, and for DETAILED feature
+requests. Everything else belongs to the Coturn Google Group:
+      
+    https://groups.google.com/forum/#!forum/turn-server-project-rfc5766-turn-server
+
+PLEASE DO NOT POST GENERAL QUESTIONS that are not about bugs or suspected
+bugs in the Github issues system. We'll be very happy to help you and provide
+all the support in the mailing list.
+
+There is also an active community of Coturn users at Stack Overflow:
+
+    https://stackoverflow.com/questions/tagged/coturn
+
+# How to provide a patch for a new feature
+
+1. If it is a major feature or a semantical change, please don't start coding
+straight away: if your feature is not a conceptual fit you'll lose a lot of
+time writing the code without any reason. Start by posting in the mailing list
+and creating an issue at Github with the description of, exactly, what you want
+to accomplish and why. Use cases are important for features to be accepted.
+Here you'll see if there is consensus about your idea.
+
+2. If in step 1 you get an acknowledgment from the project leaders, use the
+   following procedure to submit a patch:
+
+    a. Fork Coturn on github ( https://docs.github.com/en/github/getting-started-with-github/fork-a-repo )
+    b. Create a topic branch (git checkout -b my_branch)
+    c. Push to your branch (git push origin my_branch)
+    d. Initiate a pull request on github ( https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request )
+    e. Done :)
+
+3. Keep in mind that we are very overloaded, so issues and PRs sometimes wait
+for a *very* long time. However this is not lack of interest, as the project
+gets more and more users, we find ourselves in a constant need to prioritize
+certain issues/PRs over others. If you think your issue/PR is very important
+try to popularize it, have other users commenting and sharing their point of
+view and so forth. This helps.
+
+4. For minor fixes just open a pull request on Github.
+
+Thanks!