Step-by-step bear guide

_From @Makman2 on May 20, 2015 18:35_

The _writing-bears_ guide should be improved. Some notes here and there, but the major aspect I have in mind:
A step-by-step guide that implements an actual bear of our coalib (in a simplified manner), so the user sees how to operate on files and do something meaningful. In my mind is the indentation bear, that should be fairly simple for one language (So we don't take the general LanguageDefinition feature in account or whatever).

One more idea:
- A little paragraph about global bears (as demonstration: A bear that calculates the MD5 hash - or any other hash-value - from all files)

_Copied from original issue: coala-analyzer/coala#561_