Add Documentation to Help Contributors

@zonuexe

Based on some recent comments I've read in issues and pull-requests, how do you feel about adding a document specifically for programmers who want to contribute to PHP Mode? The document would describe things like:

* Any code standards you want people to follow.
* A descrption of a useful workflow, e.g.:
** Creating a branch with a useful name and types of names to avoid, like `ejmr/bugfixes`.
** Sanity checks to run before opening a pull-request, like running the test suite.
** On that topic: notes about using and (maybe more important in my opinion) expanding the test suite for all new features.
** How to seek peer-review for any non-trivial, non-tiny submissions.
* Practices to follow for writing documentation.
* And practices for writing good Git commit messages, with stuff like:
** Referencing previous commits when appropriate, especially for bug fixes.
** Including useful metadata in commit messages such as links to relevant GitHub issues.

If you believe this would be useful then I would be glad to write such a document, or at the very least contribute, on weekends where I have the energy to do so.