Clarify supported Linux distributions

[vtbassmatt]

Right now, we package for and (appear to) support only Debian. At least one early adopter is on SLES, and there are of course others. We can inventory our dependencies on platform capabilities and then decide what level of support to offer other Linux distributions.

If we choose to expand the set of distributions, some things to keep in mind:

• We may or may not produce packages for distribution $X, and should be explicit about our choices.
• We may or may not regularly test on distribution $X, and should be explicit about that.
• By offering to "support" a given distribution, we will set community expectation that we're equipped to troubleshoot on that distribution.

Later, we ought to also consider non-Linux Unixes like BSD.