local.md updated for mac os support for mongodb setup

[impiyush83]

Fixes #518

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream master branch.
• I have added necessary documentation (if appropriate)

Changes proposed in this pull request:

• Updated local.md for local installations of mongodb setup on mac

[codecov]

Codecov Report

Merging #519 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #519   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          30       30           
  Lines         896      896           
=======================================
  Hits          823      823           
  Misses         73       73

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[krtkvrm]

Issue# is missing.

[impiyush83]

@vkartik97 updated. The issue was raised by me itself, as i was blocked in mongodb installation while installing on mac. So, i raised #518 issue