Document the tsconfig.json file

[phaas]

Please add documentation for the tsconfig.json file.

I did not find any information in the Handbook and the 1.5 Release Announcement only links to the pull request.

Unfortunately the pull request does not make it clear if the example given is still valid -- the subsequent discussion indicates that there were some revisions to the proposed format.

[vvakame]

https://github.com/Microsoft/TypeScript/wiki/tsconfig.json ?

[DanielRosenwasser]

We're tracking this with microsoft/TypeScript-Handbook#2