Add link to community plugin for Spark offline store

[adchia]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[felixwang9817]

links are broken

[codecov-commenter]

Codecov Report

Merging #2279 (aca9ef0) into master (c6ef615) will increase coverage by 27.42%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2279       +/-   ##
===========================================
+ Coverage   58.60%   86.02%   +27.42%     
===========================================
  Files         116      116               
  Lines        9810     9810               
===========================================
+ Hits         5749     8439     +2690     
+ Misses       4061     1371     -2690     

Flag	Coverage Δ
integrationtests	74.81% <ø> (?)
unittests	58.60% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/feast/data_source.py	60.55% <0.00%> (+0.39%)	⬆️
sdk/python/feast/infra/online_stores/sqlite.py	95.27% <0.00%> (+1.57%)	⬆️
sdk/python/feast/errors.py	70.51% <0.00%> (+3.20%)	⬆️
sdk/python/feast/online_response.py	77.77% <0.00%> (+3.70%)	⬆️
sdk/python/feast/feature.py	90.24% <0.00%> (+4.87%)	⬆️
sdk/python/feast/infra/provider.py	89.16% <0.00%> (+5.00%)	⬆️
...k/python/feast/infra/offline_stores/file_source.py	89.21% <0.00%> (+6.86%)	⬆️
sdk/python/feast/utils.py	100.00% <0.00%> (+10.00%)	⬆️
sdk/python/feast/dqm/profilers/profiler.py	81.25% <0.00%> (+10.41%)	⬆️
sdk/python/feast/repo_config.py	88.82% <0.00%> (+11.73%)	⬆️
... and 60 more

---

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[felixwang9817]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adchia, felixwang9817

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