Updated README and collected coverage-related tests in a common dir.
There are still coverage failures resulting from either the following
scripts not being @load'd in the default bro mode:
base/frameworks/time-machine/notice.bro
base/protocols/http/partial-content.bro
base/protocols/rpc/main.bro
Or the following result in errors when @load'd:
policy/protocols/conn/scan.bro
policy/hot.conn.bro
If these are all scripts-in-progress, can we move them all to live
outside the main scripts/ directory until they're ready?
