[docs] Document "LGTM" in the lexicon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239085 91177308-0d34-0410-b5e6-96231b3b80d8

docs/Lexicon.rst

@@ -131,6 +131,10 @@ L
 **LCSSA**
     Loop-Closed Static Single Assignment Form
 
+**LGTM**
+    "Looks Good To Me". In a review thread, this indicates that the
+    reviewer thinks that the patch is okay to commit.
+
 **LICM**
     Loop Invariant Code Motion