doc: add Contributing section in README

Add Contributing section in README that describes CoC, Contributing
guide and Technical Charter.

Author: xdelaruelle

README.md

@@ -152,11 +152,20 @@ The project is also present on several social media platforms:
 * Bluesky: [@EnvModules.bsky.social][9]
 
 
-Code of conduct
----------------
+Contributing
+------------
+
+Modules project welcomes contributions of all kinds! Before submitting an
+issue or pull request, please take a moment to review our [Contributing
+guide][10]. It includes important information about issue reporting, coding
+standards, etc.
+
+Please note that Modules project has a [Code of conduct][11]. It ensures a
+respectful and inclusive environment for all contributors. By participating in
+the Modules community, you agree to abide by its rules.
 
-Please note that Modules has a [Code of conduct][10]. By participating in the
-Modules community, you agree to abide by its rules.
+This project also adheres to a [Technical charter][12], which defines its
+governance model, decision-making process, and long-term vision.
 
 
 Authors
@@ -165,13 +174,13 @@ Authors
 Modules current core developer and maintainer is Xavier Delaruelle,
 [email protected]
 
-Many thanks go to the [contributors][11] of the Modules project.
+Many thanks go to the [contributors][13] of the Modules project.
 
 
 Acknowledgments
 ---------------
 
-We would like to express our gratitude to [CEA][12] for the resources and
+We would like to express our gratitude to [CEA][14] for the resources and
 funding provided to the project over the recent years.
 
 The following people have notably contributed to Modules and Modules would not
@@ -197,6 +206,8 @@ be what it is without their contributions:
 [7]: https://x.com/EnvModules
 [8]: https://mast.hpc.social/@EnvModules
 [9]: https://bsky.app/profile/envmodules.bsky.social
-[10]: CODE_OF_CONDUCT.md
-[11]: https://github.com/envmodules/modules/graphs/contributors
-[12]: https://www.cea.fr/english
+[10]: https://modules.readthedocs.io/en/latest/CONTRIBUTING.html
+[11]: CODE_OF_CONDUCT.md
+[12]: https://modules.readthedocs.io/en/latest/CHARTER.html
+[13]: https://github.com/envmodules/modules/graphs/contributors
+[14]: https://www.cea.fr/english