Add migration hurdles section and tweak few other things in migration docs

[email protected], [email protected]
NOTRY=True

Bug: 
Change-Id: Iea7f5b3506c72e9d66a17ee1552efae5dd6a1445
Reviewed-on: https://chromium-review.googlesource.com/528938
Reviewed-by: Gabriel Charette <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/master@{#478286}
diff --git a/docs/threading_and_tasks.md b/docs/threading_and_tasks.md
index 9b28921..b29e601 100644
--- a/docs/threading_and_tasks.md
+++ b/docs/threading_and_tasks.md
@@ -67,6 +67,9 @@
 use of base::CreateSingleThreadTaskRunnerWithTraits() with a TODO against your
 bug to use base::CreateSequencedTaskRunnerWithTraits() when fixed).
 
+Detailed documentation on how to migrate from single-threaded contexts to
+sequenced contexts can be found [here](task_scheduler_migration.md).
+
 The discussion below covers all of these ways to execute tasks in details.
 
 ## Posting a Parallel Task