[clangd] Update docs to mention YCM integration and new LSP features

Reviewers: gribozavr

Reviewed By: gribozavr

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D56718

llvm-svn: 354865