Provide a 'contribution-guide' for this project

[fgheysels]

I'm currently contributing for the first time to this project, and as a non-Powershell expert, it would be nice to have some guidance available in the form of documentation.

This guide could describe what steps you need to take for certain topics:

• How to create a new module
• How to create a new script (create a ps1 file, make the required modifications in the psd1 and psm1 files, .... )
• How to create a unit test
• How to run the unit-tests (I'm currently struggling on this step for instance, when running Invoke-Pester -Path <file>, I'm always faced with the error 'Module is not a Script module>'.

[stijnmoreels]

• How contributors with more privileges are able to release/prerelease the project