Update IDEA setup instructions

Configuring the annotation processor is done automatically as of IDEA 2020.1.

Run configs for 1.14+ versions should now be generated automatically once the
referenced PR is merged (or when using a local build of it).

versions/common.gradle

@@ -120,6 +120,7 @@ archivesBaseName = "replaymod"
 if (FABRIC) {
     minecraft {
         refmapName = 'mixins.replaymod.refmap.json'
+        autoGenIDERuns = true
     }
 } else {
     minecraft {