Do let user know how to update openneuro client

[yarikoptic]

I do not know what other means to install it exist, but I as a nobody in npm world simply forget how to do that every time I see this message. Hinting on a typical way it is installed/updated IMHO would be useful

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 44.44%. Comparing base (8f78abb) to head (a6f9299).

Files with missing lines	Patch %	Lines
packages/openneuro-client/src/client.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3256      +/-   ##
==========================================
- Coverage   45.68%   44.44%   -1.24%     
==========================================
  Files         595      551      -44     
  Lines       38015    36394    -1621     
  Branches     1171     1171              
==========================================
- Hits        17366    16175    -1191     
+ Misses      20452    20022     -430     
  Partials      197      197              

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