Document --platform flag in README.md

[imjasonh]

Followup on #38

[codecov-commenter]

Codecov Report

Merging #211 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
+ Coverage   37.15%   37.23%   +0.07%     
==========================================
  Files          33       33              
  Lines        1405     1402       -3     
==========================================
  Hits          522      522              
+ Misses        793      790       -3     
  Partials       90       90              

Impacted Files	Coverage Δ
pkg/commands/apply.go	0.00% <0.00%> (ø)
pkg/commands/create.go	0.00% <0.00%> (ø)
pkg/commands/resolve.go	0.00% <0.00%> (ø)

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[jonjohnsonjr]

I have seen a lot of folks referencing the --platform=amd64 syntax, which won't work (has been corrected in this PR). If people don't like having to --platform=linux/amd64, we should talk about that now before this implementation ossifies.

[imjasonh]

I have seen a lot of folks referencing the --platform=amd64 syntax, which won't work (has been corrected in this PR). If people don't like having to --platform=linux/amd64, we should talk about that now before this implementation ossifies.

I think it's fine the way it is. I don't expect people to really use anything besides "all platforms" or "my current platform" in practice.