readme: add a how to contribute chapter

Waiting the wiki, add a chapter to inform how to contribute.
This chapter is inspired from zephyr documentation.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>

Author: arnopo

README.md

@@ -244,3 +244,43 @@ libmetal sleep APIs provide getting delay execution implementation.
 This API is for compiler dependent functions.  For this release, there is only
 a GCC implementation, and compiler specific code is limited to atomic
 operations.
+
+## How to contribute:
+As an open-source project, we welcome and encourage the community to submit patches directly to the project. As a contributor you  should be familiar with common developer tools such as Git and CMake, and platforms such as GitHub.
+Then following points should be rescpected to facilitate the review process.
+
+### Licencing
+Code is contributed to OpenAMP under a number of licenses, but all code must be compatible with version the [BSD License](https://github.com/OpenAMP/libmetal/blob/master/LICENSE.md), which is the license covering the OpenAMP distribution as a whole. In practice, use the following tag instead of the full license text in the individual files:
+
+    ```
+    SPDX-License-Identifier:    BSD-3-Clause
+    ```
+### Signed-off-by
+Commit message must contain Signed-off-by: line and your email must match the change authorship information. Make sure your .gitconfig is set up correctly:
+
+    ```
+    git config --global user.name "first-name Last-Namer"
+    git config --global user.email "yourmail@company.com"
+    ```
+### gitlint
+Before you submit a pull request to the project, verify your commit messages meet the requirements. The check can be performed locally using the the gitlint command.
+
+Run gitlint locally in your tree and branch where your patches have been committed:
+
+      ```gitlint```
+Note, gitlint only checks HEAD (the most recent commit), so you should run it after each commit, or use the --commits option to specify a commit range covering all the development patches to be submitted.
+
+### Code style
+In general, follow the Linux kernel coding style, with the following exceptions:
+
+* Use /**  */ for doxygen comments that need to appear in the documentation.
+
+The Linux kernel GPL-licensed tool checkpatch is used to check coding style conformity.Checkpatch is available in the scripts directory.
+
+To check your \<n\> commits in your git branch:
+   ```
+   ./scripts/checkpatch.pl --strict  -g HEAD-<n>
+
+   ```
+### Send a pull request
+We use standard github mechanism for pull request. Please refer to github documentation for help.