Update Issue Mod documentation

[erictleung]

• Change easy label to first-timers-only
• Add note on accepting your own PRs

[abhisekp]

:sparkles: :smile_cat: :+1: LGTM :smiley: :+1: :sparkles:

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Merging in 5 minutes

If anyone wants to STOP me, please comment.

STOP Merging :hand:

# STOP Merging :hand: