Update README.md

Updated install since this is now a registered package.

Author: logankilpatrick

README.md

@@ -13,13 +13,6 @@ After that, you will need to add a `webhook` to your app such that you can send
 julia> ]
 (v1.3) pkg> add Slack
 ```
-_Note that this will not work till the package is registered._
-
-For the time being do:
-```julia
-julia> ]
-(v1.3) pkg> add https://github.com/logankilpatrick/Slack.jl.git
-```
 
 ## Usage