slack importer: Add README.md.
ADD
zulip/integrations/slack/README.md
+# Slack Data Import Tool
+
+The slack import tool has been shifted to the Zulip server repository.
+The data can be converted from slack data format to zulip data format
+using the command `./manage.py convert_slack_data`.
+
+The procedure to use this would be:
+
+1. `./manage.py convert_slack_data <slack_zip_file> <realm_name> --output <output_dir>`.
+2. To import this converted data into a new Zulip instance, use
+ `./manage.py import --destory-rebuild-database <output_dir>`.
+3. To import this converted data into an existing Zulip instance (with multiple realms)
+ use `./manage.py import --import-into-nonempty <output_dir>`.

slack importer: Add README.md.