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?

> ---------------------------------------------------------------
> Thanks! We'll be in touch soon.

The issue does not name a documentation file or test. Review the existing Android keystore and signature instructions first, then verify the proposed command and its binary SHA-1 to Base64 step; it is done when the documentation states the confirmed command and answers that question.

Escrito por el modelo de indexación a partir del texto del issue.