Add reference to IR framework

Author: dstogov

CONTRIBUTING.md

@@ -206,6 +206,9 @@ locations.
        ├─ libmbfl/                  # Forked and maintained in php-src
        ├─ unicode_data.h            # Generated by `ext/mbstring/ucgendat/ucgendat.php`
        └─ ...
+    └─ opcache/
+       └─ jit/
+          └─ ir/                    # Bundled part of IR framework https://github.com/dstogov/ir
     └─ pcre/
        ├─ pcre2lib/                 # https://www.pcre.org/
        └─ ...