Add git guide

[bettio]

A practical guide about how to make commits and PRs for this project.

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

[UncleGrumpy]

This looks very helpful, but we should also make sure it gets included in the published documentation by adding it to doc/CmakeLists.txt around here:

AtomVM/doc/CMakeLists.txt

Line 28 in 6f66fa4

file(COPY ${CMAKE_SOURCE_DIR}/C_CODING_STYLE.md DESTINATION ${CMAKE_CURRENT_BINARY_DIR}/src)

It will be good to have this published, then the other AtomVM.org repos can just add a link to it in their readme or contribution guidelines.