Create CONTRIBUTING.md

Cleans up some of the process focused content in the README and also adds nice message to github issue tracker, etc.
Link to current performance tests in readme
Update issue filing guidelines
Optimize compiler opcode call
Allow passing depths to _child
FAQ tweaks
Merge mustache compatibility sections
Expose escapeExpression on the root object
Refactor content blocks to ignore lines

We can simplify our previous standalone determination logic by merging content blocks again.

Fixes #854
Rev runtime version