Add clarification for description formatting

[echasnovski]

Currently the only rule for description is "A description MUST immediately follow the colon and space after the type/scope prefix. The description is a short summary of the code changes, e.g., fix: array parsing issue when multiple spaces were contained in string.".

Besides "be short", there is no specification on its formatting. I would like to suggest expanding this with (at least) two rules which are already followed in all examples and most real world usages:

• First word MUST NOT be capitalized.
• MUST NOT end with punctuation.

If any of this is a welcome change, I am happy to do a PR.