update README wit-cli instructions

[corinnekrych]

• Please describe what your change is about. README.doc update
• What issues does it solve? (Use autoreferencing for this) update stale instruction
• If you are still working on the change please prefix the pull request title with "WIP"

@kwk I couldn't find the command to create a workitemtype, if you type in:

$bin/wit-cli create 
Available Commands:
  codebase           
  label              
  query              
  space              
  space-codebases    
  space-iterations   
  tracker            
  trackerquery       
  work-item-comments 
  work-item-link     
  workitems 

workitemtype is not listed

[codecov[bot]]

Codecov Report

Merging #2356 into master will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2356      +/-   ##
==========================================
+ Coverage   70.18%   70.19%   +0.01%     
==========================================
  Files         171      171              
  Lines       16919    16919              
==========================================
+ Hits        11874    11877       +3     
+ Misses       3893     3892       -1     
+ Partials     1152     1150       -2

Impacted Files	Coverage Δ
controller/workitem.go	79.22% <0%> (+0.42%)	:arrow_up:

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[kwk]

@corinnekrych the work item type creation/update/delete part is not something we expose via HTTP. The same is true for space templates. You can only read them or use them to create work items at this point.

[corinnekrych]

[test]