add links to clang-tidy and clangd in discourse

Author: vbvictor

clang-tools-extra/README.txt

@@ -8,8 +8,10 @@ Clang frontend.  These tools are kept in a separate "extra" repository to
 allow lighter weight checkouts of the core Clang codebase.
 
 All discussion regarding Clang, Clang-based tools, and code in this repository
-should be held using the standard Clang forum:
+should be held using the standard Clang forums:
   https://discourse.llvm.org/c/clang
+  https://discourse.llvm.org/c/clang/clang-tidy/71
+  https://discourse.llvm.org/c/clang/clangd/34
 
 Code review for this tree should take place on Github:
   https://github.com/llvm/llvm-project/pulls?q=label%3Aclang-tools-extra