Update README.md: KDevPlatform merge, plugins, new manual, GitLab

1 job for !152 with update-readme in 60 minutes (queued for 37 minutes and 28 seconds)
latest detached
Status Job ID Name Coverage
  Build
failed #61998
linux kf5-qt5 qt5.12

01:00:00

 
Name Stage Failure
failed
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
kdeinit5: sending SIGHUP to children.
klauncher: Exiting on signal 1
The X11 connection broke (error 1). Did the X11 server die?
$ python3 -u $CI_TOOLING/helpers/print-lcov-results.py --product $PRODUCT --project $PROJECT --branchGroup $BRANCH_GROUP --platform $PLATFORM
kdeinit5: sending SIGTERM to children.
kdeinit5: Exit.
Uploading artifacts for failed job
Pulling docker image gitlab/gitlab-runner-helper:x86_64-a998cacd ...
ERROR: Job failed: execution took longer than 1h0m0s seconds