Define keystore signature calculation algorithm

### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [x] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Expected/desired behavior
> Please provide exact Android keystore signature generation command in docs.

`keytool -exportcert -alias [aliasname] -keystore [your app keystore path] | openssl sha1 -binary | openssl base64`

Please confirm, if above command is correct.
Specifically, should base64 be generated from binary form of sha1?

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