docs: run python library solely

[GaoxiangLuo]

Add a README file to illustrate how one can run python library manually without engaging with the control plane.

type	file	changes
added	lib/python/flame/README.md	a tutorial
modified	lib/python/flame/examples/mnist/aggregator/config.json	indent
modified	lib/python/flame/examples/mnist/trainer/config.json	indent

[codecov-commenter]

Codecov Report

Merging #187 (ae38992) into main (c4a02eb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   21.21%   21.21%           
=======================================
  Files          35       35           
  Lines        1560     1560           
=======================================
  Hits          331      331           
  Misses       1219     1219           
  Partials       10       10           

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[myungjin]

lgtm; thanks!