Documentation: Added contribution guidelines

One of the most annoying issue with bug reports is that they are incomplete, nonspecific, inconsistent and/or wooly.

For that matter, GitHub provides a simple way [contribution guidelines](https://github.com/blog/1184-contributing-guidelines), by adding `CONTRIBUTION.md` file at repo root which would result in:

![GitHub contribution guidelines graphics](https://camo.githubusercontent.com/71995d6b0e620a9ef1ded00a04498241c69dd1bf/68747470733a2f2f6769746875622d696d616765732e73332e616d617a6f6e6177732e636f6d2f736b697463682f6973737565732d32303132303931332d3136323533392e6a7067)

Since Web Essentials is a hub of heterogeneous features pertaining for miscellaneous technologies, sometimes its really hard to chase after the user to get the complete context of the bug report. Next, it takes time for a new contributor to master the entire Web Essentials source. A good bug report would keep the discussion focused and hence prevent the new contributor rambling around different areas of code and eventually get lost.

Hopefully, this will improve quality of issue reported in WE in future and attract more contributors.