[firebase_crashlytics] Add missing documentation for android installation in README.md

[areille]

Description

This PR adds the missing documentation for firebase_crashlytics plugin installation on android.

Related Issues

• relates to [firebase_crashlytics] No errors are reported in the Firebase console #1376
• closes [firebase_crashlytics] incomplete documentation #1153

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[areille]

@googlebot I signed it!

[TahaTesser]

Hi @augustinreille
You've some conflicts and "CLAs are signed, but unable to verify author consent"
Please create a new PR with no conflicts with CLA with verify author consent
Thank you

[axel-op]

I added this in #2288