readme: Link man pages in root README

[ZirixCZ]

Mentioned here #367 (comment)

Currently it is possible to view the man pages either through installing them or by finding the correct file in the project. This PR links the man pages directly in the root README.

[joggee-fr]

I was thinking of a more explicit sentence in the "Building and running Cage" section. For example, "The available command line options and environment variables are listed in the dedicated man page.", something like that.

[ZirixCZ]

That is what I wanted to write. The man page doesn't only include command line arguments however. Though it might be good to put it right next to the guide on how to run cage after it's been built. Something like: You can run Cage by running ./build/cage APPLICATION. List of available command line options and environment variables is listed in the dedicated man page.

[emersion]

Or should it be in the first section, near the wiki and project page links? (Since it's useful for users installing from their distro package too instead of building from source)

[emersion]

Thanks!