Add @snoopl to reference docs

[NHDaly]

I just noticed that @snoopl isn't in the documentation at all. I know it's not very widely used, but it'd be nice to refer to it here at least. :)

@snoopl was originally added in #135.

[codecov]

Codecov Report

Merging #245 (d4bae7c) into master (5ad2f24) will increase coverage by 0.64%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   84.65%   85.29%   +0.64%     
==========================================
  Files           9        9              
  Lines        1590     1578      -12     
==========================================
  Hits         1346     1346              
+ Misses        244      232      -12     

Impacted Files	Coverage Δ
src/parcel_snoopi.jl	93.77% <0.00%> (-0.89%)	⬇️
src/parcel_snoopi_deep.jl	88.07% <0.00%> (+0.81%)	⬆️
src/invalidations.jl	86.51% <0.00%> (+1.27%)	⬆️
src/parcel_snoopc.jl	78.37% <0.00%> (+1.80%)	⬆️

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[timholy]

Seems fine to me! Some more documentation might increase its utility, but TBH I haven't played with it enough to know.