Add license file to release packages

[farshidtz]

Describe the feature

The release packages include the binary distribution and the configuration file only. The license file is missing.

$ wget https://github.com/aler9/rtsp-simple-server/releases/download/v0.20.0/rtsp-simple-server_v0.20.0_linux_amd64.tar.gz
...
Saving to: ‘rtsp-simple-server_v0.20.0_linux_amd64.tar.gz’
...
$ tar -xvf rtsp-simple-server_v0.20.0_linux_amd64.tar.gz 
rtsp-simple-server
rtsp-simple-server.yml

The same goes with the Docker image, the license isn't included in there.

I believe the license needs to be bundled in all packages to comply with the following MIT license term:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

[aler9]

Added in v0.20.2

[github-actions]

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