Update README.md

[btrautmann]

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.

Note: I didn't file an issue for this since it's such a small change.

Hey there! So you want to contribute to FirebaseUI? Before you file this pull request, follow these steps:

• Read the contribution guidelines.
• Run ./gradlew check to ensure the Travis build passes.
• If this has been discussed in an issue, make sure to mention the issue number here. If not, go file an issue about this to make sure this is a desirable change.
• If this is a new feature please co-ordinate with someone on FirebaseUI-iOS to make sure that we can implement this on both platforms and maintain feature parity.

[samtstern]

Thanks for this!