[devhelp] Finally write a HACKING file

[Sébastien Wilmet <swilmet src gnome org>]

commit 31be174d7660c1f4cf668ec24f8c5f278322b111
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Thu May 24 16:35:17 2018 +0200

    Finally write a HACKING file
    
    Better late than never.
    
    I'm sure there are some missing exceptions for the C code conventions.
    That section can be improved over time, and it can be a good thing to
    reduce the number of exceptions, by applying GNU indent, uncrustify or
    something else.

 HACKING | 25 +++++++++++++++++++++++++
 README  |  5 +++++
 2 files changed, 30 insertions(+)
---
diff --git a/HACKING b/HACKING
new file mode 100644
index 00000000..ad28c86f
--- /dev/null
+++ b/HACKING
@@ -0,0 +1,25 @@
+How to contribute to Devhelp
+============================
+
+Devhelp is hosted on the GNOME GitLab instance, you can fork the
+repository and then do a merge request:
+
+    https://gitlab.gnome.org/GNOME/devhelp
+
+Read the following wiki page to know the conventions for the commit messages:
+
+    https://wiki.gnome.org/Git/CommitMessages
+
+C code conventions
+------------------
+
+Devhelp follows the Linux kernel coding style, with some exceptions:
+- Indentation: 8 spaces, not tabs.
+- One space before *each* opening parenthesis.
+- Multi-line comments must be like this:
+        /* First line.
+         * Second line.
+         */
+
+See also:
+https://developer.gnome.org/programming-guidelines/unstable/
diff --git a/README b/README
index 95049b4f..4303999d 100644
--- a/README
+++ b/README
@@ -5,6 +5,11 @@ The Devhelp web page:
 
     https://wiki.gnome.org/Apps/Devhelp
 
+How to contribute
+-----------------
+
+See the HACKING file.
+
 Dependencies
 ------------