Gh-1083: Add deprecation notice to README

[t92549]

Related Issue

• Resolve Archive gaffer-tools #1083

[codecov-commenter]

Codecov Report

Merging #1084 (ff8012d) into develop (d4f0498) will not change coverage.
Report is 6 commits behind head on develop.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #1084   +/-   ##
==========================================
  Coverage      45.29%   45.29%           
  Complexity       101      101           
==========================================
  Files             25       25           
  Lines            892      892           
  Branches          72       72           
==========================================
  Hits             404      404           
  Misses           458      458           
  Partials          30       30           

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