clang-format vs plugin: include the license

The vsix installer will show the license when it starts, and it
would print an annoying message when there was none.

While we're here, add a MoreInfoUrl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214237 91177308-0d34-0410-b5e6-96231b3b80d8