DOC: Add usage instructions for running from deno.land

[effigies]

The current README is overly developer-focused. This adds docs that will appear on the deno.land landing page (https://deno.land/x/bids_validator) that tell users how to use what's there.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -2.32% ⚠️

Comparison is base (5ec0b3d) 85.71% compared to head (0adb522) 83.40%.
Report is 9 commits behind head on master.

❗ Current head 0adb522 differs from pull request most recent head 04d3fd2. Consider uploading reports for the commit 04d3fd2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1770      +/-   ##
==========================================
- Coverage   85.71%   83.40%   -2.32%     
==========================================
  Files         130       92      -38     
  Lines        6575     3893    -2682     
  Branches     1524     1270     -254     
==========================================
- Hits         5636     3247    -2389     
+ Misses        833      548     -285     
+ Partials      106       98       -8     

see 38 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sappelhoff]

thanks!

[nellh]

Looks good except we should update the --legacy option docs since it was removed.