[docs][android] readme: improved “Releasing Updates” and “Multi-Deployment Testing” c…

[sergey-akhalkov]

…hapters

CodePush updates should be tested in modes other than Debug due to in Debug mode React Native app always downloads JS bundle generated by packager, so JS bundle downloaded by CodePush does not apply.

Relates #614 and #682

[msftclas]

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[buptkang]

+*NOTE: CodePush updates should be tested in modes other than Debug mode. In Debug mode, React Native app always downloads JS bundle generated by packager, so JS bundle downloaded by CodePush does not apply.

[sergey-akhalkov]

@buptkang, thank you, I've applied the correction