CONTRIBUTE.md: mention the out-of-tree build test too

Every pull request submitted will automatically be tested in several different

ways. Every pull request is verfied that:

 - ... the code still builds, warning-free, on Linux and macOS, with both

 - ... it still builds, warning-free, on Linux and macOS, with both

   clang and gcc

 - ... the code still builds fine on Windows with several MSVC versions

 - ... the code still builds with cmake on Linux, with gcc and clang

 - ... the code follows rudimentary code style rules

 - ... it still builds fine on Windows with several MSVC versions

 - ... it still builds with cmake on Linux, with gcc and clang

 - ... it follows rudimentary code style rules

 - ... the test suite still runs 100% fine

 - ... the release tarball (the "dist") still works

 - ... the code coverage doesn't shrink drastically

 - ... it builds fine in-tree as well as out-of-tree

 - ... code coverage doesn't shrink drastically

If the pull-request fails one of these tests, it will show up as a red X and

you are expected to fix the problem. If you don't understand whan the issue is