create CONTRIBUTING.md with pointer to mezcla version

[tomasohara]

Please create a new file in the root directory: CONTRIBUTING.md. Add a pointer to the mezcla version.

Mention that the style is not as strict as mezcla; however, the spirit of the guidelines should be kept in mind. This especially holds for for new python scripts, unless they are throwaway code such as during perl-to-python conversions.

[aveeyan]

CONTRIBUTING.md has been added to the root directory, with a link to the mezcla library. Please refer to this latest commit from add-contributing-md branch