The apt-get update command requires --allow-releaseinfo-change

[mirkobunse]

This PR updates the README to describe a successful installation of system level dependencies.

I expect this improvement to solve the issues #33 and #53 because I had a similar issue that is solved by this PR.

Background: When running apt-get update -y && apt-get install -y git, I obtained the following error messages, hinting at a problem with release info changes.

Reading package lists...
E: Repository 'http://deb.debian.org/debian buster InRelease' changed its 'Suite' value from 'stable' to 'oldoldstable'
E: Repository 'http://deb.debian.org/debian buster-updates InRelease' changed its 'Suite' value from 'stable-updates' to 'oldoldstable-updates'
E: Repository 'http://security.debian.org/debian-security buster/updates InRelease' changed its 'Suite' value from 'stable' to 'oldoldstable'

By adding --allow-releaseinfo-change to apt-get update, this problem is now solved and system level dependencies are correctly installed.

Similar issues have been solved in the same way elsewhere.