Add disclaimer and link to maintained fork in README

[leoravagnani]

@phucledien as this is not maintained anymore, consider adding a disclaimer to the README and a link pointing to this fork https://github.com/UeharaYou/HiddenBar

Every now and then there's people asking about the future of this project and there's multiple issues being opened that will never get a response...

Thanks!

[yippie]

Where did you get that this repo is no longer maintained? If it is not, there definitely should be an update to the readme

[leoravagnani]

Last release 2 years ago, multiple unmerged PRs, maintainer doesn't answer any of the issues for a really long while, etc etc etc,
This very issue was opened 3 months ago and nada. 🤷🏻‍♂️

Apart from the fork above, there's Ice as another free alternative actively maintained.

See #284