Document the flutter cache

Author: valotas

README.md

@@ -20,12 +20,15 @@ pipelines:
  caches:
  - gradle
  - gradlewrapper
+ - flutter
  script:
  - echo "Building APK..."
+ - flutter upgrade
  - flutter doctor
  - flutter -v build apk
 
 definitions:
  caches:
  gradlewrapper: ~/.gradle/wrapper
+ flutter: /opt/flutter
 ```