Update README.md (#1506)

Kotlin users will need to use `kapt` instead of `annotationProcessor` or `GlideApp` will not be generated. Could be a bit clearer with this comment in the docs.

Author: Brandon Trautmann

storage/README.md

@@ -30,6 +30,7 @@ to your `app/build.gradle` file:
 ```groovy
 // Find the latest Glide releases here: https://goo.gl/LpksbR
 implementation 'com.github.bumptech.glide:glide:4.x'
+// If you're using Kotlin (and therefore, kapt), use kapt instead of annotationProcessor
 annotationProcessor 'com.github.bumptech.glide:compiler:4.x'
 ```