Update documentation to cover JSONPath compliance

[hiltontj]

There should be a note about compliance included in the documentation. This may be at the README level, but could also go into the docs in its own section; in doing, the paragraph in the opening of the crate documentation could potentially be moved out.

This was part of #41 originally, but I moved it out as I want to wait for the spec to be standardized, and the compliance test suite to be finalized before asserting compliance.