Update usage doc to reflect limitations

[kincl]

The limitations on the main page talk about not using the reloader but
the usage example enables the reloader, this patch simply updates the
exmaple to reflect the limitation.

[coveralls]

Pull Request Test Coverage Report for Build 70

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 96.104%

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Files with Coverage Reduction	New Missed Lines	%
flask_mqtt/init.py	2	96.1%

Totals
Change from base Build 68:	0.0%
Covered Lines:	148
Relevant Lines:	154

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💛 - Coveralls

[stlehmann]

Thanks for the PR :)