docs: Add DEVELOPMENT.md [gapic-generator-python]

[miraleung]

No description provided.

[codecov]

Codecov Report

Merging #876 (4c5f67c) into master (7c185e8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #876   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        27    +1     
  Lines         1608      1697   +89     
  Branches       328       347   +19     
=========================================
+ Hits          1608      1697   +89     

Impacted Files	Coverage Δ
gapic/samplegen_utils/types.py	100.00% <ø> (ø)
gapic/samplegen_utils/utils.py	100.00% <ø> (ø)
gapic/utils/case.py	100.00% <ø> (ø)
gapic/utils/reserved_names.py	100.00% <ø> (ø)
gapic/generator/generator.py	100.00% <100.00%> (ø)
gapic/samplegen/samplegen.py	100.00% <100.00%> (ø)
gapic/schema/api.py	100.00% <100.00%> (ø)
gapic/schema/metadata.py	100.00% <100.00%> (ø)
gapic/schema/wrappers.py	100.00% <100.00%> (ø)
gapic/utils/__init__.py	100.00% <100.00%> (ø)
... and 3 more

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[busunkim96]

Thanks @miraleung!