[Doc] Link to doc/contributing.rdoc from CONTRIBUTING.md (#4266)

* [Doc] Link to doc/contributing.rdoc from CONTRIBUTING.md

[ci skip]

* [Doc] Add path prefix `doc/`

#4266 (comment)

* [Doc] Prefer relative path in CONTRIBUTING.md

This patch by nobu (Nobuyoshi Nakada), thank you!

ref: #4266 (comment)

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

CONTRIBUTING.md

@@ -1,4 +1,5 @@
-Please see the [official issue tracker] and wiki [HowToContribute].
+Please see the [official issue tracker], [doc/contributing.rdoc] and wiki [HowToContribute].
 
 [official issue tracker]: https://bugs.ruby-lang.org
+[doc/contributing.rdoc]: doc/contributing.rdoc
 [HowToContribute]: https://bugs.ruby-lang.org/projects/ruby/wiki/HowToContribute