More documentation on code coverage/hpc

[ezyang]

Our code coverage documentation is a bit woeful, only mentioning the existence of --enable-coverage. Here is the bare minimum:

• The info in http://stackoverflow.com/questions/28416827/cabal-and-hpc-and-errors-when-running-tests-with-code-coverage
• More detail about how coverage integrates with test suites, calling out explicitly that other use cases of coverage are not directly supported (cabal and integration tests or how to get code coverage for binaries #3902)
• Where the reports/tix files go