Telling Git to ignore a local change without unversioning the file (#219)

Ignoring local changes can be really handy for creds/credfile templates, I've found it particularly handy with Discord bot development where the configuration file can have an expected structure and expects you to add tokens to that config file

README.md

@@ -107,6 +107,7 @@ All commands should work for at least git version 2.13.0. See the [git website](
     - [I want to remove a file from Git but keep the file](#i-want-to-remove-a-file-from-git-but-keep-the-file)
     - [I want to revert a file to a specific revision](#i-want-to-revert-a-file-to-a-specific-revision)
     - [I want to list changes of a specific file between commits or branches](#i-want-to-list-changes-of-a-specific-file-between-commits-or-branches)
+    - [I want Git to ignore changes to a file without deleting it](#i-want-git-to-ignore-changes-to-a-specific-file)
   - [Configuration](#configuration)
     - [I want to add aliases for some Git commands](#i-want-to-add-aliases-for-some-git-commands)
     - [I want to add an empty directory to my repository](#i-want-to-add-an-empty-directory-to-my-repository)
@@ -1414,6 +1415,20 @@ Same goes for branches:
 $ git diff master:path_to_file/file staging:path_to_file/file
 ```
 
+### I want Git to ignore changes to a specific file
+
+This works great for config templates or other files that require locally adding credentials that shouldn't be committed.
+
+```sh
+$ git update-index --assume-unchanged file-to-ignore
+```
+
+Note that this does *not* remove the file from source control - it is only ignored locally. To undo this and tell Git to notice changes again, this clears the ignore flag:
+
+```sh
+$ git update-index --no-assume-unchanged file-to-stop-ignoring
+```
+
 ## Configuration
 
 ### I want to add aliases for some Git commands