Add contribution guidelines to PMM GitHub - Style guide

We have external contributors who need documentation on how to get started with developing with PMM. This ticket is about technical instructions, unless you want to add more background information.

Minimum DoD in order of importance

A (useful -> see list below) CONTRIBUTING.md exists somewhere in a main PMM-related repo (https://github.com/percona/pmm/tree/PMM-2.0 ) and can be linked to from other PMM-repos  

1. Any code style/contribution quality expectations we have

   1. https://confluence.percona.com/display/PMM/Common+tech%2C+libraries+and+approaches 

   2. https://confluence.percona.com/display/PMM/Frontend+development 

Additional information

Example CONTRIBUTING.md: https://github.com/atom/atom/blob/master/CONTRIBUTING.md

GitHub docs: https://docs.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors

More information: http://mozillascience.github.io/working-open-workshop/contributing/