docs : updated readme.md

[hemantjadon]

updated readme.md for documentation of styling --iron-icon-width and --iron-icon-height.

In reference to #87.

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[hemantjadon]

I signed it!

[googlebot]

CLAs look good, thanks!

[hemantjadon]

Yeah you are right. These properties are exposed by iron-icon but it is not mentioned anywhere in the docs of paper-icon-button how to style these.

[notwaldorf]

Thanks, this is perfect!