Update contributors guide

This is mostly the same text from the new pull request template.

Author: fragglet

.github/CONTRIBUTING.md

@@ -1,7 +1,13 @@
-Because of spam, an automated bot will automatically close all pull requests
-opened on this repository. If you've found a legitimate bug in the source code,
-you can open an issue to report it.
+This project is intended to contain high quality implementations of data structures and algorithms. Do not submit code that you just wrote for a class assignment.
 
-This project isn't a dumping ground for random bits of C code you've written.
-That goes double if you're being instructed to send pull requests as part of a
-competition or a school class.
+If you are planning to add a new module (data structure or algorithm), you may want to open an issue to discuss your plan with the project maintainer first. This will avoid wasted effort on your part. 
+
+If you are planning to submit a pull request, please ensure that your change:
+* Is written in the C programming language. 
+* Conforms to the project's style guidelines. 
+* Does not duplicate existing code already present. 
+* Passes all unit test checks (existing code). Run `make check` to confirm. 
+* Adds unit tests (new code) with at least 95% coverage. Build with `configure –enable-coverage` to confirm. 
+* Is properly documented using Doxygen comments. 
+
+Automated continuous integration checks will fail for many of the above requirements if they are not satisfied.