docs: Add link to the sdk for README generation regarding Stream Maps

[visch]

Docs seems right 🤷

[codecov]

Codecov Report

Merging #823 (8ac114d) into main (c82c71c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #823   +/-   ##
=======================================
  Coverage   80.29%   80.29%           
=======================================
  Files          34       34           
  Lines        3410     3410           
  Branches      679      679           
=======================================
  Hits         2738     2738           
  Misses        499      499           
  Partials      173      173           

Impacted Files	Coverage Δ
singer_sdk/helpers/capabilities.py	91.37% <ø> (ø)

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[edgarrmondragon]

lgtm 🙂