Add installation method on Fedora using snaps

[robvdl]

Thanks @johncurley for adding this and testing

[codecov-io]

Codecov Report

Merging #139 into master will not change coverage.
The diff coverage is n/a.

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##           master     #139   +/-   ##
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  Coverage   91.72%   91.72%           
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  Files          45       45           
  Lines        1293     1293           
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  Hits         1186     1186           
  Misses        107      107

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[robvdl]

Several things need to be added:

• The Fedora SNAP method seems to work fine with SELinux and doesn't encourage the user to disable it, so it might actually be best to recommend the SNAP method first even on Fedora, rather than last.
• The Fedora COPR method is missing the lxd-client dependency

@johncurley mentioned he might be able to send a patch to the COPR repository maintainer to sort the missing SELinux rules out. But it does seem that the SNAP package is better maintained than the COPR method so we really should be mentioning it first instead of last.

[robvdl]

So it turns out the COPR method isn't even working anymore? Something about a missing dependency, we should really be listing the SNAP way first then.