Update version reference in README

[Guilsimi]

Version documented in the README points to a bug-affected version. It should be updated to 0.0.20-SNAPSHOT so users don’t have to check the commit history to find the correct version.

Here:

dependencies {
  implementation('com.openpix:sdk-java:0.0.13-SNAPSHOT')
}

// This version must be updated to 0.0.20-SNAPSHOT.
}