Add link to bugzilla query of unassigned enhancements. Specific open

projects like 'port glibc to llvm' or 'improve nightly tester', should
have an unassigned enhancement bug opened for them so that they can be
tracked more easily. Open projects should only list generic projects
like 'compile programs with the LLVM compiler' or 'write a new backend
for target'.

llvm-svn: 12273