Add doc for sending patches to GCC

Author: GuillaumeGomez

doc/sending-gcc-patch.md

@@ -0,0 +1,26 @@
+This guide explains what to do to send a GCC patch for review.
+
+All the commands are supposed to be run in the folder where you cloned GCC.
+
+```bash
+./contrib/gcc-changelog/git_check_commit.py
+```
+
+When there is no more errors, generate the git patch:
+
+```bash
+git format-patch -1 `git rev-parse --short HEAD`
+```
+
+Then you can run the remaining checks using:
+
+```bash
+contrib/check_GNU_style.sh 0001-your-patch.patch
+```
+
+When you have no more errors, you can send the `.patch` file to GCC by sending an
+email to `[email protected]` and to the relevant GCC mailing lists
+depending on what your patch changes. You can find the list of the mailing lists
+[here](https://gcc.gnu.org/lists.html).
+
+You can find more information about "contributing to GCC" [here](https://gcc.gnu.org/contribute.html).