Document requirements for example apps

[bryophyta]

Adds a requirements.txt file in ./examples/, and some documentation of dependencies in the README.

Specifically, the direct dependencies at the time of writing are Textual (for all of the example apps) and httpx (for dictionary.py) the file includes both of these, plus their dependencies, according to pip freeze. The requirements.txt file in this PR is the output of:

python -m venv venv
venv bin activate
pip install textual
pip install httpx
pip freeze -r requirements.txt

See #1090