d1c826d1 ("fuse: increase signal/noise in log messages",
https://github.com/hanwen/go-fuse/pull/249) changed log to use
abbreviations in the name of improving signal/noise ratio. However it
might be not evident offhand what an abbreviation means.

Add a short reference in the readme with the summary on how to read the
log and a short example.

The summary is incomplete - for example I don't describe Fh and other
attributes that d1c826d1 did not change. I propose we review logging
more, e.g. consider doing `Fh X` -> `fX` and maybe similarly for other
fields, and while doing so, also populate/update the summary and example
in the readme. In other words it is good to document current state, but
it is not fixed in stone and can be evolved.