API tutorials: including the RS API

In your build.gradle:

Adding the repository

repositories {
    maven {
        name = "refinedstorage"
        url = "https://repo.raoulvdberge.com/"

Adding the dependency

dependencies {
    compile "refinedstorage:refinedstorage:VERSION_HERE+:api"

Replace VERSION_HERE with the version you want. The latest one at the time of writing is 1.5.34-11 for Minecraft 1.12.2 (May 2018).

For a list of all versions, see: https://repo.raoulvdberge.com/refinedstorage/refinedstorage/.

Which build/version do I choose?

Choose the latest build of the version you want to support.