Add CONFIG.md

https://github.com/harfbuzz/harfbuzz/issues/1652

README.md

diff --git a/README.md b/README.md
index ce0a04d..ce3e84d 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,8 @@
 
 For build information, see [BUILD.md](BUILD.md).
 
+For custom configurations, see [CONFIG.md](CONFIG.md).
+
 For test execution, see [TESTING.md](TESTING.md).
 
 Documentation: https://harfbuzz.github.io