Add missing `public_addresses` field to README

[linked0]

This adds the missing public_addresses field to README which is needed to run a validator.

[codecov[bot]]

Codecov Report

Merging #3271 (a75147f) into v0.x.x (d782bfe) will increase coverage by 0.93%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           v0.x.x    #3271      +/-   ##
==========================================
+ Coverage   87.76%   88.70%   +0.93%     
==========================================
  Files         164      165       +1     
  Lines       16963    17072     +109     
==========================================
+ Hits        14888    15143     +255     
+ Misses       2075     1929     -146     

Flag	Coverage Δ
integration	42.93% <ø> (-0.05%)	:arrow_down:
unittests	87.47% <ø> (-0.02%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/flash/Node.d	80.67% <0.00%> (-0.22%)	:arrow_down:
source/agora/consensus/Ledger.d	97.38% <0.00%> (-0.15%)	:arrow_down:
source/agora/consensus/state/Ledger.d	90.26% <0.00%> (ø)
source/agora/consensus/protocol/Nominator.d	90.56% <0.00%> (ø)
source/agora/node/main.d	44.85% <0.00%> (ø)
source/agora/node/Validator.d	91.45% <0.00%> (+0.04%)	:arrow_up:
source/agora/network/Manager.d	80.43% <0.00%> (+1.86%)	:arrow_up:
source/agora/node/FullNode.d	76.16% <0.00%> (+2.78%)	:arrow_up:
source/agora/node/Config.d	68.61% <0.00%> (+3.64%)	:arrow_up:
source/agora/network/Client.d	90.90% <0.00%> (+8.33%)	:arrow_up:
... and 4 more

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