[doc] Add Slack API token generation info

This commit adds documentation for creation of
API token for the Slack backend.
Slack apps can be used to generate an API token.
A script to generate the token is also linked.

This change was necessary since the documentation
was lacking this information. Additionally, legacy
tokens are deprecated since May 5, 2020. Slack
proposes the use of Slack apps as the new alternative.
Ref: https://api.slack.com/changelog/2020-02-legacy-test-token-creation-to-retire

Signed-off-by: Animesh Kumar <[email protected]>

Author: animeshk08

README.md

@@ -327,6 +327,18 @@ $ perceval rss 'https://blog.bitergia.com/feed/'
 ```
 
 ### Slack
+
+Slack backend requires an API token for authentication. Slack apps can be
+used to generate and configure this API token. The scopes required by a Slack
+app for the backend are `channels:history`, `channels:read` and `users:read`.
+To know more about Slack apps and its integration please refer the
+[Slack apps documentation](https://api.slack.com/start/overview).
+For more information about the scopes required by a Slack app please refer the
+[Scopes and permissions documentation](https://api.slack.com/scopes).
+
+The following [script](https://gist.github.com/valeriocos/de31324625a3fab32449cf5d43b24075)
+can also be used to generate an OAuth2 token to access the Slack API.
+
 ```
 $ perceval slack C0001 --from-date 2016-01-12 -t abcedefghijk
 ```