Add note about generating source maps to load() and transform()

[tjenkinson]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers: n/a

Description

Small change to docs

[codecov]

Codecov Report

Merging #3252 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3252   +/-   ##
=======================================
  Coverage   92.74%   92.74%           
=======================================
  Files         170      170           
  Lines        5925     5925           
  Branches     1793     1793           
=======================================
  Hits         5495     5495           
  Misses        224      224           
  Partials      206      206

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[lukastaegert]

Awesome, thanks!