Update catalogapi Readme

[Castronova]

Update the catalogapi readme to describe the project more accurately and provide links to the schema documents.

[Castronova]

Moving out of April release.

[horsburgh]

@Castronova - I edited the main readme file to provide a bit more detail about what the catalog is and what we are trying to accomplish. If you want to link to the schema documents then a new section should be added to the main readme that describes where those schemas are located. I'll let you handle that once the schemas have been moved into the repo.

[horsburgh]

@Castronova - I added this to the next milestone because I think we want to have the GitHub repositories fully sorted out as soon as we can so we don't have pieces spread all over.