slack-import: Explain current implementation about threads.
slack-import: Explain current implementation about threads.

rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,kou/zulip,punchagan/zulip,rht/zulip,rht/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip