README update due to recent improvements

[akolotov]

Recently two new parameters were introduced:

• APP_STYLES allows to specify a theme prepared for particular deployment (https://github.com/poanetwork/bridge-ui/pull/192)
• REACT_APP_TITLE allows to specify the page title (https://github.com/poanetwork/bridge-ui/pull/201)

[netlify[bot]]

Deploy preview for kind-kilby-95344f processing.

Building with commit ca343bf57cedee155e489f7fe229c6e22c8fbfb5

https://app.netlify.com/sites/kind-kilby-95344f/deploys/5c9f392f7f76a20007cf98c4

[coveralls]

Pull Request Test Coverage Report for Build 445

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 4.985%

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Totals
Change from base Build 443:	0.0%
Covered Lines:	74
Relevant Lines:	1365

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💛 - Coveralls