Add app.add_routes() method

[asvetlov]

A part of #2766 issue

[asvetlov]

It's a shame but app.add_subapp() was still documented as app.router.add_subapp.

[codecov-io]

Codecov Report

Merging #2787 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2787      +/-   ##
==========================================
+ Coverage   97.99%   97.99%   +<.01%     
==========================================
  Files          39       39              
  Lines        7377     7379       +2     
  Branches     1296     1296              
==========================================
+ Hits         7229     7231       +2     
  Misses         47       47              
  Partials      101      101

Impacted Files	Coverage Δ
aiohttp/web_app.py	98.92% <100%> (+0.01%)	⬆️

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new