docs: Cross reference contributing

.md points to .html on latest
.rst points to .md on main (not that it will work before this is merged)

CONTRIBUTING.md

@@ -5,6 +5,8 @@ first time contributing to an open source project, please take a look at the
 following guide:
 https://opensource.guide/how-to-contribute/#orienting-yourself-to-a-new-project.
 
+Information about the Yosys coding style is available on our Read the Docs:
+https://yosys.readthedocs.io/en/latest/yosys_internals/extending_yosys/contributing.html.
 
 # Using the issue tracker
 

docs/source/yosys_internals/extending_yosys/contributing.rst

@@ -1,6 +1,14 @@
 Contributing to Yosys
 =====================
 
+.. note::
+
+   For information on making a pull request on github, refer to our
+   |CONTRIBUTING|_.
+
+.. |CONTRIBUTING| replace:: :file:`CONTRIBUTING.md`
+.. _CONTRIBUTING: https://github.com/YosysHQ/yosys/CONTRIBUTING.md
+
 Coding Style
 ------------