Update i_u_m_update.md

docs/i_u_m_update.md

@@ -13,6 +13,17 @@ If it needs to, it will ask you how you wish to proceed.
 Merge errors will be reported.
 Some minor conflicts will be auto-corrected (in favour for the mailcow: dockerized repository code).
 
+### I forgot what I changed before running update.sh
+
+See `git log --pretty=oneline | grep -i "before update"`, you will have an output similar to this:
+
+```
+22cd00b5e28893ef9ddef3c2b5436453cc5223ab Before update on 2020-09-28_19_25_45
+dacd4fb9b51e9e1c8a37d84485b92ffaf6c59353 Before update on 2020-08-07_13_31_31
+```
+
+Run `git diff 22cd00b5e28893ef9ddef3c2b5436453cc5223ab` to see what changed.
+
 ### Options
 
 ```