Add instructions for updating the version number

[rohan20]

A lot of Flutter users are publishing their apps to the Play store but finding a way to update the version number and name isn't straightforward.

Hence, this answer is gaining more attention every week: https://stackoverflow.com/a/56970752/5066615

Even though the instructions are directly available in pubspec.yaml, most users like me couldn't find it easily initially.

Hence, this PR uses the same words from the pubspec.yaml on updating the version number of the app.

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[rohan20]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[sfshaza2]

This is great, @rohan20! Thanks so much! I have some minor editing nits, but those can be fixed later.