Add documentation about y and x dimensions for custom readers

[djhoese]

After fixing parts of the tropomi_l2 reader in #1031 I noticed that the custom reader documentation doesn't explicitly mention that dimensions should be named y and x. This PR adds that information.

• Fully documented

[coveralls]

Coverage increased (+0.02%) to 87.29% when pulling c522656 on djhoese:doc-custom-reader-dims into c80a510 on pytroll:master.

[codecov]

Codecov Report

Merging #1032 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1032      +/-   ##
==========================================
+ Coverage   87.27%   87.29%   +0.01%     
==========================================
  Files         188      188              
  Lines       28739    28742       +3     
==========================================
+ Hits        25082    25089       +7     
+ Misses       3657     3653       -4

Impacted Files	Coverage Δ
satpy/readers/aapp_l1b.py	13.5% <0%> (-0.16%)	⬇️
satpy/readers/ami_l1b.py	97.39% <0%> (-0.09%)	⬇️
satpy/tests/reader_tests/test_tropomi_l2.py	94.05% <0%> (+0.24%)	⬆️
satpy/readers/tropomi_l2.py	94.87% <0%> (+0.58%)	⬆️

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[mraspaud]

You write that if it's at least 2 dimensions, it should also have x, so we can't have y and z only ? (eg caliop)

[djhoese]

Good point. I'll update that in a bit.

[mraspaud]

LGTM