Update README.md file

[aleks-rope]

Users should be able to install Apollo by following the instructions found in README.md.

It seems the current README.md is a bit outdated specifically in terms of using gradle.

[KeepBotting]

What specifically is outdated? Were you having trouble with a particular gradle command?

[garyttierney]

It seems that Apollo doesn't build on Gradle 6.x and the documentation still points to using system Gradle rather than the Gradle wrapper.

[aleks-rope]

What specifically is outdated? Were you having trouble with a particular gradle command?

As @garyttierney mentioned the documentation isnt specifying to use the gradle wrapper. Perhaps we can update the command or point the user to https://docs.gradle.org/current/userguide/gradle_wrapper.html