DOC replace gitter link in README

[MarcoGorelli]

In this note

pandas/README.md

Line 169 in 2a2daf7

Feel free to ask questions on the [mailing list](https://groups.google.com/forum/?fromgroups#!forum/pydata) or on [Gitter](https://gitter.im/pydata/pandas).

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

[Mohd-Farhan]

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

[radojicic23]

take

[radojicic23]

I've fixed this issue.