Document process for bringing changes over from master

[SpacemanPaul]

... and update constraints.txt

Proposed changes

• Document process for bringing changes over from master

• update constraints.txt (which I wouldn't have to be doing if I'd documented it last time.

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

Codecov Report

Patch coverage: 91.66% and project coverage change: -0.01 ⚠️

Comparison is base (c97bf11) 85.18% compared to head (b17b314) 85.18%.

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1478      +/-   ##
===============================================
- Coverage        85.18%   85.18%   -0.01%     
===============================================
  Files              130      130              
  Lines            14352    14358       +6     
===============================================
+ Hits             12226    12231       +5     
- Misses            2126     2127       +1     

Impacted Files	Coverage Δ
datacube/utils/documents.py	98.28% <83.33%> (-0.33%)	⬇️
datacube/drivers/postgis/_core.py	98.98% <100.00%> (+0.02%)	⬆️
datacube/drivers/postgres/_core.py	99.03% <100.00%> (+<0.01%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.