Clarify automatic use vs automatic install

[codecov[bot]]

Codecov Report

Merging #193 (1e7dd47) into master (7dd556d) will increase coverage by 3.65%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   70.45%   74.10%   +3.65%     
==========================================
  Files           3        3              
  Lines         396      421      +25     
==========================================
+ Hits          279      312      +33     
+ Misses        117      109       -8     

Impacted Files	Coverage Δ
src/ImageMagick.jl	83.55% <0.00%> (+8.72%)	:arrow_up:
src/libmagickwand.jl	67.39% <100.00%> (+1.65%)	:arrow_up:
src/precompile.jl	76.92% <0.00%> (-4.16%)	:arrow_down:

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[timholy]

Thanks!