Add tip to have Bedrock Linux report itself as Bedrock Linux

[paradigm]

Software often query lsb-release to detect the distro it is running under.
Bedrock Linux has two options in these situations:

• Report itself as Bedrock Linux. This would be preferable for things like
  screenfetch, the Steam survey, crash reporting tools which phone home the
  distro they were running under, etc.
• Report itself as the local stratum. This is required for tools like
  add-apt-repository (i.e. Ubuntu PPA) to work properly, as they will abort
  if they fail to detect a distro they recognize.

Right now, Bedrock Linux does the latter, as having tools like
add-apt-repository "just work" is a high priority. However, instructions
should be available to reconfigure things so that Bedrock Linux reports itself
as Bedrock Linux for users who do not plan to utilize add-apt-repository.