Documentation on github actions workflows

[scottyhq]

We've recently revamped the github action workflows to use 'composite workflows', and tried to consolidate as much as possible. The README.md, could be updated for clarity #45 (comment)

Also per discussion, I think that README.md could also have a 'Best Practices' section for modifying the workflows. For example, as discussed here , it's sometimes desirable for the default test.yaml workflow to have access to credentials. This was the case for SnowEx hackweek so that tutorial notebooks had access to NASA Logins to access remote data. In this case the default 'test.yaml' workflow can be modified to require a label being set by an administrator before it is run:

on:
  pull_request_target:
    types: [labeled, synchronize]
jobs:
  build-and-test:
    # This workflow accesses secrets and checks out a PR, so only run if labelled
    # https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
    if: contains(github.event.pull_request.labels.*.name, 'preview')
    runs-on: ubuntu-20.04
    steps:
    - uses: actions/checkout@v2
      with:
        ref: ${{ github.event.pull_request.head.sha }}

https://github.com/snowex-hackweek/website/blob/ffd063b9cfba8f4c92642466544d147e776db9ae/.github/workflows/test.yaml

[scottyhq]

closed by #67