Update instructions for using PackageCompiler.jl

[f-ttok]

I found that PackageCompiler.jl had been updated and procedures to compile packages had been changed.
This PR gives some modifications to instructions to compile Gadfly.jl in the document.

In my environment, PackageCompiler.jl added by ]add PackageCompiler (not ]dev PackageCompiler ) works well, so I think the first caveat may not be necessary.

PackageCompiler is a work in progress, and the most recent tagged release
sometimes (currently as of this writing) does not work. Hence the dev
instead of add command to install.

[codecov-commenter]

Codecov Report

Merging #1467 into master will decrease coverage by 0.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1467      +/-   ##
==========================================
- Coverage   90.23%   89.44%   -0.79%     
==========================================
  Files          39       39              
  Lines        4197     4416     +219     
==========================================
+ Hits         3787     3950     +163     
- Misses        410      466      +56     

Impacted Files	Coverage Δ
src/poetry.jl	90.74% <0.00%> (-5.26%)	⬇️
src/theme.jl	65.07% <0.00%> (-4.42%)	⬇️
src/geom/label.jl	96.26% <0.00%> (-2.74%)	⬇️
src/geom/beeswarm.jl	90.00% <0.00%> (-2.65%)	⬇️
src/aesthetics.jl	77.77% <0.00%> (-2.06%)	⬇️
src/misc.jl	65.06% <0.00%> (-2.06%)	⬇️
src/Gadfly.jl	75.88% <0.00%> (-1.92%)	⬇️
src/color_misc.jl	60.60% <0.00%> (-1.47%)	⬇️
src/geom/rectbin.jl	80.00% <0.00%> (-1.09%)	⬇️
src/guide/keys.jl	82.69% <0.00%> (-0.97%)	⬇️
... and 17 more

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[bjarthur]

thanks! looks great.