Add precommit section to upgrade guide

docs/upgrade_guides/5.0.0.md

@@ -8,6 +8,9 @@ Related documentation:
 * [isort 5.0.0 changelog](https://timothycrosley.github.io/isort/CHANGELOG/#500-penny-july-4-2020)
 * [isort 5 release document](https://timothycrosley.github.io/isort/docs/major_releases/introducing_isort_5/)
 
+!!! important - "If you use pre-commit remove seed-isort-config."
+    If you currently use pre-commit, make sure to see the pre-commit section of this document. In particular, make sure to remove any `seed-isort-config` pre-step.
+
 ## Migrating CLI options
 
 ### `--dont-skip` or `-ns`
@@ -49,3 +52,22 @@ isort has completely rewritten its logic for placing modules in 5.0.0 to ensure
 The TL;DR of which is that isort has now changed from `default_section=FIRSTPARTY` to `default_section=THIRDPARTY`. If you all already setting the default section to third party, your config is probably in good shape.
 If not, you can either use the old finding approach with `--magic-placement` in the CLI or `old_finders=True` in your config, or preferably, you are able to remove all placement options and isort will determine it correctly.
 If it doesn't, you should be able to just specify your projects modules with `known_first_party` and be done with it.
+
+## Migrating pre-commit
+
+### seed-isort-config
+
+If you have a step in your precommit called `seed-isort-config` or similar, it is highly recommend that you remove this. It is unnecessary in 5.x.x, is guaranteed to slow things down, and worse can conflict with isort's own module placement logic.
+
+### isort pre-commit step
+
+isort now includes an optimized precommit configuration in the repo itself. To use it you can replace any existing isort precommit step with:
+
+```
+  - repo: https://github.com/timothycrosley/isort
+    rev: master
+    hooks:
+      - id: isort
+```
+
+under the `repos` section of your projects `.pre-commit-config.yaml` config.