(#239): Add cross compiling document for win32

Fixes #239
7 jobs for master in 33 minutes and 25 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #331104
fedora test

00:08:55

 
  Lint
manual #331106
allowed to fail manual
clippy
manual #331105
allowed to fail manual
rustfmt
passed #331107
tarball

00:11:06

 
  Cross Distro
passed #331110
debian_i386:testing

00:13:23

passed #331109
allowed to fail
debian:testing

00:07:57

failed #331108
allowed to fail
fedora:rawhide

00:08:10

 
Name Stage Failure
failed
fedora:rawhide Cross Distro
make[2]: *** [Makefile:1452: check] Error 2
make[1]: Leaving directory '/builds/GNOME/librsvg/_build'
make[1]: *** [Makefile:1414: check-recursive] Error 1
make: *** [Makefile:1710: check] Error 2
Uploading artifacts...
_build/tests/*.log: found 9 matching files
_build/tests/output/: found 743 matching files
Uploading artifacts to coordinator... ok id=331108 responseStatus=201 Created token=TBAay-fb
ERROR: Job failed: exit code 1