Create README.md in python-builder folder (#7054)

Author: tyndria

python-builder/README.md

@@ -0,0 +1,11 @@
+# Summary
+
+`seldonio/python-builder` image is used as container for [`V1 Python Lint`](.github/workflows/python_lint.yml) and [`V1 Python Tests`](.github/workflows/python_tests.yml) Github Workflows.
+
+Also this image is used to build python package and push it to PyPi, see `run_python_builder` command from the [root Makefile](Makefile).
+
+# How to push new image
+
+You could either use `push_to_registry` command from the [Makefile](./Makefile) or you could:
+1. Run `gh workflow list` to get the id of the `Build & Push the Python Builder Image` workflow.
+2. Run `gh workflow run <ID> -f docker-tag=<DOCKERTAG> --ref <BRANCH FROM WHICH YOU'D LIKE TO RUN THE WORKGLOW>`