docs: add vscode linter instructions to CONTRIBUTING.md

[suddjian]

SUMMARY

Something in the updated eslint/babel config isn't able to function without this extra config, since the frontend code is not in the project root.

TESTING INSTRUCTIONS

@geido to verify the solution works.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #17045 (0278b0e) into master (a6173f1) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17045      +/-   ##
==========================================
- Coverage   76.92%   76.92%   -0.01%     
==========================================
  Files        1031     1031              
  Lines       55151    55151              
  Branches     7498     7498              
==========================================
- Hits        42426    42424       -2     
- Misses      12473    12475       +2     
  Partials      252      252              

Flag	Coverage Δ
hive	81.47% <ø> (ø)
javascript	70.88% <ø> (ø)
mysql	81.92% <ø> (ø)
postgres	81.93% <ø> (ø)
presto	81.79% <ø> (-0.01%)	⬇️
python	82.43% <ø> (-0.01%)	⬇️
sqlite	81.60% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/db_engine_specs/presto.py	89.95% <0.00%> (-0.42%)	⬇️

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[geido]

LGTM. Thanks for finding the solution to this annoying problem!