Add comment on how to speed up the CI with better caching

.github/workflows/linux_ci.yml

@@ -34,6 +34,10 @@ jobs:
  uses: actions/cache@v2
  with:
  path: ${{ env.pythonLocation }}
+ # The cache currently misses at every change in the Python files to ensure
+ # that the code changes to Haystack are not being cached along with the dependencies.
+ # TODO for the tests refactoring: We could speed up the process by caching only 
+ # the dependencies and installing Haystack again separately for each test runner.
  key: linux-${{ hashFiles('**/*.py') }}-${{ hashFiles('setup.cfg') }}-${{ hashFiles('pyproject.toml') }}
 
  - name: Install dependencies

.github/workflows/windows_ci.yml

@@ -35,6 +35,10 @@ jobs:
  uses: actions/cache@v2
  with:
  path: ${{ env.pythonLocation }}
+ # The cache currently misses at every change in the Python files to ensure
+ # that the code changes to Haystack are not being cached along with the dependencies.
+ # TODO for the tests refactoring: We could speed up the process by caching only 
+ # the dependencies and installing Haystack again separately for each test runner.
  key: windows-${{ hashFiles('**/*.py') }}-${{ hashFiles('setup.cfg') }}-${{ hashFiles('pyproject.toml') }}
 
  - name: Install Pytorch on windows