Create CONTRIBUTING.md

This adds a nice link on the issue and pr forms and is mostly pinched from https://github.com/update/update. I added clear references to C4 and our CoC and we can add more stuff like coding guidelines later on.

CONTRIBUTING.md

@@ -0,0 +1,32 @@
+# Contributing to LibreTime
+
+First and foremost, thank you! We appreciate that you want to
+contribute to LibreTime, your time is valuable, and your 
+contributions mean a lot to us.
+
+**What does "contributing" mean?**
+
+Creating an issue is the simplest form of contributing to a
+project. But there are many ways to contribute, including 
+the following:
+
+* Updating or correcting documentation
+* Feature requests
+* Bug reports
+
+Before opening an issue, please:
+
+* read and be prepared to adhere to our [code of conduct](https://github.com/LibreTime/code-of-conduct/blob/CODE-OF-CONDUCT.md)
+* understand that we follow the standardized [C4 development process](https://rfc.zeromq.org/spec:42/C4/)
+* [search for existing duplicate or closed issues](https://github.com/LibreTime/libretime/issues?utf8=%E2%9C%93&q=is%3Aissue)
+* clearly state the problem you would like to solve in a meaningful way
+* be prepared to follow up on issues by providing additional information as requested by a maintainer or contributor helping you out
+
+For bug reports, please provide the following details:
+
+* **version**: what version of LibreTime you were using when you experienced the bug?
+* **reduced test case**: the minimum amount of detail needed to reproduce the bug
+* **error messages**: please paste any error reports into the issue or a gist
+
+Please wrap all code and error messages in [markdown code 
+fences](https://help.github.com/articles/creating-and-highlighting-code-blocks/).