[DISCUSS] Git commit style guide

[Janardhan <ja...@apache.org>]

Hi all,

*Why do we need one?*

We have a good number of new contributors, and a doc[1] to document
our styling is important!. To communicate the nuances of why
we need to be more specific in description of commit messages.

*Tags proposed?*

Based on the commit history so far, (the description can be improved!)
1. [MINOR] - Small changesets with additional functionality
2. [SYSTEMDS-#] - The tag value is found from SystemDS jira issue tracker.
3. [DOC] - Changes to the documentation
4. [HOTFIX] - Introduce a fix in the hot running system.

*Why was there no commit guide previously?*

Previously the teams were from the same org (such as spark.tc,),
and individual contributors are mentored in the beginning
where it is easier to easily communicate style guide, commit
philosophy more subtly. People used to share their personal commit
notes [2][3].

[1] https://github.com/apache/systemds/pull/1231
[2] http://niketanblog.blogspot.com/2017/10/?m=0
[3]
https://gist.github.com/deroneriksson/e0d6d0634f3388f0df5e#integrate-pull-request-with-one-commit-into-apache-master

Thank you,
Janardhan