Add documentation step to add task guide

[jeswan]

This PR adds a step to the Add a task guide to document the added task in the supported tasks chart.

@wh629 Can you check I correctly documented arct? Also, please add the task in #1152 to the task chart.

[codecov]

Codecov Report

Merging #1154 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1154   +/-   ##
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  Coverage   56.91%   56.91%           
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  Files         133      133           
  Lines        9696     9696           
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  Hits         5518     5518           
  Misses       4178     4178           

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[zphang]

We should mention adding to the downloader as well. (Also, we need to clean up the organization of constants for task names for the downloader - they're in two very separate files currently.)

(Even more ambitiously, we could consider moving the downloading (+eval) logic to the task files themselves.)