doc: init Commit Messages Guideline

Based on the discussion irc://ni.r/#retiolum at 2017-06-26

doc/Commit_Messages_Guideline.md

@@ -0,0 +1,53 @@
+# Commit Messages Guideline
+
+Commits SHOULD have the following format:
+
+```
+<namespace?> <component>: <change>
+
+<rationale>
+
+(<reference-name>: <reference-id>)?
+```
+
+## `<namespace>`
+Defines where the change took place. This can be omitted if the
+namespace is `krebs`. Namespaces may be shortened to one to four characters (
+lassulus -> lass, makefu -> make, tv -> tv, shared -> sha)
+
+## `<component>`
+Name of the component which was touched. `component` is
+rather fuzzy and may mean different things, just choose what would fit best.
+
+Here are a numbers of samples for defining the component:
+
+* Change `gum` in `krebs/3modules/makefu/default.nix`: `gum.r: change ip`
+* Change `prepare.sh` in `krebs/4libs/infest`: `infest: prepare stockholm ISO`
+* Remove `concat` in `krebs/5pkgs`: `concat: RIP`, this commit may like some `<rationale>`
+* Update `types` in `krebs/3modules`: `lib/types: add managed bool to host type`
+* Change host `gum` in `makefu/1systems/gum`: `ma gum.r: add taskserver`
+* Change `tinc` module in `krebs/3modules`: `tinc module: add option enableLegacy`
+
+## `<rationale>`
+Describe some trivia why the commit was done:
+```
+whatsupnix: init
+
+Import from https://github.com/NixOS/nix/issues/443#issuecomment-296752535
+```
+
+## `<reference>`
+Defines external resouces related to the commit:
+```
+Closes: #123533
+CVE: CVE-2016-00001
+URL: https://example.com/CVE-2016-00001
+```
+
+## Remarks
+As a general rule of thumb you can check out: https://www.slideshare.net/TarinGamberini/commit-messages-goodpractices
+Of course the pattern not always fits perfectly (for example for refactoring),
+just apply some common sense and define a useful commit message,
+like `refactor krebs.setuid`.
+
+