Fix #348 app.view() is not described in the README

[seratch]

Summary

This pull request fixes #348 by adding the explanation about app.view handlers on README.

In addition, I wanted to propose having another example of app.action with callback_id patterns. Referring to this table may be helpful to understand the whole.

There is one more task related to this.

• cherry-pick the commit after merging this and make a PR to @slack/bolt@next branch

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #410 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #410   +/-   ##
=======================================
  Coverage   81.33%   81.33%           
=======================================
  Files           7        7           
  Lines         525      525           
  Branches      150      150           
=======================================
  Hits          427      427           
  Misses         70       70           
  Partials       28       28

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