Update READMEs

Updated the main `README` to include information about the Cloud SDK and list the Go components currently in development.

Updated `gcloud_apis/README` to include installation and usage instructions.

Added [GFM](https://help.github.com/articles/github-flavored-markdown/) syntax.
This is not necessary. All source starts generated. Only if they change the set of discovery docs or make some code changes will gen.bash have to be run.