Fix Readme text for apks

[mariobehling]

Fix the information in the Readme. It still refers to apks and apk branch.

apk: This branch contains many apk files, that are automatically generated on the merged pull request a) debug apk b) release apk
There are multiple files in the apk branch of the project, this branch consists of all the APK files and other files that are relevant when an APK is generated.
Once a pull request is merged, the previous APK branch is deleted and a new APK branch is created.
If a PR is merged in development branch then the new APKs for the development branch are generated whereas the APKs corresponding to the master branch are not regenerated and simply the previously generated files are added.
version: This branch stores the version information for the APKs (versionName and versionCode). This is used in our workflows for automatic versioning wherein the next version information is automatically fetched from this branch and used for building APKs.

This has been changed a while back and the branch is called "app" now.

[NKD52]

Hello, I would like to work on this issue if thats possible
Thank you