strbuf: add strbuf_tolower function

diff --git a/RelNotes b/RelNotes
index 7b78eb9715d5ba10b65deee6bb565d93ed3cc365..e50885caa1043665dfe2d995e0a2f7a7a7bb51ca 120000 (symlink)
--- a/RelNotes
+++ b/RelNotes
@@ -1 +1 @@
-Documentation/RelNotes/1.9.3.txt
\ No newline at end of file
+Documentation/RelNotes/2.0.0.txt
\ No newline at end of file