feat(build): fluidattacks#1108 document contributing

Add new sections to the contributing.md file to help newcomers.

- Running your local changes
- Adding yourself to the mailmap
- Validating commit message
- DCO
- Adding and rendering documentation

Signed-off-by: Robin Quintero <rohaquinlop301@gmail.com>

docs/src/contributing.md

@@ -38,6 +38,40 @@ Examples:
 - [feat(build): #252 aws secrets from env](https://github.com/fluidattacks/makes/commit/1c9f06a809bd92d56939d5809ce46058856fdf0a)
 - [feat(build): #232 make parallel utils](https://github.com/fluidattacks/makes/commit/99e9f77482a6cbc9858a7a928a91a8a8aa9ff353)
 
+### Running your local changes
+
+Once you have finished your changes you need to add them to the staging area.
+You should use `git add`, and add the files.
+
+After that, just run `m . <job>` and if everything goes well you won't receive any error message, if you do, then keep working, you're almost there!
+
+### Adding yourself to the mailmap
+
+When contributing is important to add yourself to the `.mailmap` file.
+This file is lexicographically sorted, so you must to add you with this in mind.
+
+You can test it running `m . /lintGitMailMap`.
+
+### Validating commit message
+
+Once you have your commit ready to be pushed, you can verify if your commit have the proper structure with `m . /lintGitCommitMsg`.
+
+### DCO
+
+Contributors sign-off that they adhere to these requirements by adding a `Signed-off-by` line to commit messages.
+
+```
+This is my commit message
+
+Signed-off-by: Full Name <email>
+```
+
+Make sure that the commit's author matches with this information and the `.mailmap` file information.
+
+### Adding and rendering documentation
+
+When contributing in documentation use `m . /docs/dev` to render it.
+
 ### The legal side of contributions
 
 All of the code