Write a GUIDELINES.md file to provide recommendations on contributions

[lbroudoux]

As a follow-up of #324, we decided that a file providing guidances for contributors should be initialized at the repository root. This fies will hold and present "rules" on how to organize and present documentation for Microcks: from general organization to layout consideration to code block formatting, etc...

[krishi-agrawal]

hi laurent, I would like to work on this issue

[divyansh-prks]

i am also ready to pick this issue because i think it requires multiple assignees

[yada]

For code block formatting. See:
#324 (comment)
and
#361

[yada]

Please see a first draft to initiate this task:
https://github.com/yada/microcks.io/blob/master/GUIDELINES.md

Feedback, changes... welcome ;)

[yada]

README file updated as well, see: https://github.com/yada/microcks.io/blob/master/README.md