Add a list of common producers to the README

[coveralls]

Pull Request Test Coverage Report for Build e4cdb7fe803437f68f5d6176b8a766c767e69a10-PR-309

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 94.035%

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Files with Coverage Reduction	New Missed Lines	%
lib/broadway/topology/producer_stage.ex	1	86.18%
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Totals
Change from base Build dbb50a92e567824314ff891c96c87d9c94d67c33:	0.2%
Covered Lines:	599
Relevant Lines:	637

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💛 - Coveralls

[josevalim]

We moved those here: https://hexdocs.pm/broadway/introduction.html

Should we bring those sections as is to the README?

[josevalim]

Another option is to add a "See the docs for detailed how-tos and supported producers" to the end of the Introduction section linking to the page above.

[whatyouhide]

@josevalim I didn't know this was in the docs, nice. I pointed the README to the docs 🙃