add a contributing style-guide document

CONTRIBUTING.md

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+# Contriuting
+
+In order to contribute to this Handbook please be aware of the following style guide:
+
+## Markdown
+
+* We are using GitHub flavoured Markdown.
+* The biggest header in chapters should be `h3`, or `###`
+* Please ensure a (single) empty line before and after headers
+* Each sentences should be put on its own line
+
+## Glossary
+
+The word "Jail" (as concept) should always be written as with upper-case J.
+The word "jail" (command or system function) should always be written as `jail`.
+To distinguish between the command and the function you can use the man page section: e.g.: `jail(8)`
+
+## CoC
+