README: contributing ref [skip ci]

g4s8 · web-flow · commit 4311a1618b6e · 2017-10-04T19:09:29.000+04:00
diff --git a/README.md b/README.md
@@ -48,3 +48,6 @@ MatcherAssert.assertThat(
     new HmMimeHasSubType("bmp")
 );
 ```
+
+## Contributing
+If you are interested in contributing please refer to [CONTRIBUTING.md](CONTRIBUTING.md)
