Add the list of code owners to the top level of the LLVM source tree to

Add the list of code owners to the top level of the LLVM source tree to
hopefully make it more visible.  Adjust the web-docs to have a link to
this file rather than the list itself.  I described code owners as also
being gatekeepers for their part of the code, which I think is true but
isn't in the code owner explanation on the web page.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160776 91177308-0d34-0410-b5e6-96231b3b80d8