Add more metadata from Sentinel-1 Prodict Specification

[alexamici]

Related to #21

[codecov-commenter]

Codecov Report

Merging #25 (265e2dd) into main (cd0e56e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   91.63%   91.66%   +0.03%     
==========================================
  Files           7        7              
  Lines         502      504       +2     
  Branches       59       59              
==========================================
+ Hits          460      462       +2     
  Misses         25       25              
  Partials       17       17              

Impacted Files	Coverage Δ
xarray_sentinel/conventions.py	100.00% <100.00%> (ø)
xarray_sentinel/sentinel1.py	92.22% <100.00%> (ø)

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[alexamici]

Ready to be merged for me.

[alexamici]

OK, I merge this myself assuming nothing is controversial so I can sync the README