Update CrossAccount.md

[kevinemoore]

Docs Update

Adding instructions to enable subscribing to existing object notifications (SNS topic) from a Quilt stack in another account.

TODO

• [x] Documentation

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a351cb1) 35.49% compared to head (f641d2f) 35.49%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3752   +/-   ##
=======================================
  Coverage   35.49%   35.49%           
=======================================
  Files         711      711           
  Lines       31244    31244           
  Branches     4674     4674           
=======================================
  Hits        11091    11091           
  Misses      18999    18999           
  Partials     1154     1154           

Flag	Coverage Δ
api-python	91.22% <ø> (ø)
catalog	10.61% <ø> (ø)
lambda	86.26% <ø> (ø)

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