Pandas DOC replace gitter link in README

In this note

https://github.com/pandas-dev/pandas/blob/2a2daf787c5bf4015729f6660d189389d3d7dc47/README.md?plain=1#L169

let's replace the second link with one which links to Slack https://pandas.pydata.org/docs/dev/development/community.html?highlight=slack#community-slack

Comment From: Mohd-Farhan

Hello, I am interrested in it. Can u assign this to me?

Comment From: radojicic23

take

Comment From: radojicic23

I've fixed this issue.