Expand documentation in Readme.md
Expand documentation in Readme.md

Add some supporting documentation regarding Closure for those unfamiliar., Also substantiate details for "files in this directory" - including them all will result in a project that does not compile as some are used for test cases and aren't part of the core library.
protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript