Add beginner onboarding README and teaching comments

[mariaob1201]

What

Makes this repo a proper "initiate people to programming with OpenLambda" example, as the canonical thing to point newcomers to (it's already referenced by the getting-started guide as the ol admin install <git-url> demo).

Changes

• README.md (new): a beginner walkthrough — what a lambda is, the f(event) contract, prerequisites, install-from-git, invoke with curl, and a "now change it" exercise (greet by name from event), plus pointers to the examples/ dir and worker docs.
• f.py: unchanged behavior (return 'hello'), now with comments explaining the three-line contract for first-time readers.

Verification

f.py still installs and returns "hello":

./ol admin install https://github.com/open-lambda/hello-lambda-example.git
curl -X POST localhost:5000/run/hello-lambda-example -d ''   # -> "hello"

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