Document usage as a git-driver in README

Author: alerque

README.md

@@ -33,6 +33,7 @@
 - Display the first n lines of the diff output with the `--head` option
 - Display the last n lines of the diff output with the `--tail` option
 - Execute the diff with an external diff tool using the `--external` option
+- Use in Git as a diff-driver to be the default diff tool for fonts
 
 Run `fdiff --help` to view all available options.
 
@@ -101,6 +102,30 @@ $ fdiff [OPTIONS] [PRE-FONT FILE URL] [POST-FONT FILE FILE PATH]
 
 ⭐ **Tip**: Remote git repository hosting services (like Github) support access to files on different git branches by URL.  Use these repository branch URL to compare fonts across git branches in your repository.
 
+#### As Git's diff driver for fonts
+
+Git can be configured to automatically use a specific tool to diff specific file types. The are to steps to complete to use `fdiff` as the default diff output for font files.
+
+1. Tell Git how to run the tool. As with most git configuration options this may be set for a repository (`--local`), for your user (`--global`), or system (`--system`). We recommend setting this at the user level. Assuming `fdiff` is available in your path, this setting should do the trick:
+
+        git config --global diff.fdiff.command 'fdiff -c --git'
+
+    This will write it to your `$HOME/.gitconfig` file looking like this:
+
+    ```gitconfig
+    [diff "fdiff"]
+        command = fdiff -c --git
+    ```
+
+    Of course you may also edit it there.
+
+2. Tell Git to use that specific tool for supported file types. This may also be done at multiple places. Each repository may have it's own `.gitattributes` file, a user may have one setting global defaults in `$XDG_HOME/git/attributes`, or their may be a system wide default file. Whichever file you choose to place this is, the lines to add are the same:
+
+    ```gitattributes
+    .otf   diff=fdiff
+    .ttf   diff=fdiff
+    ```
+
 ### Options
 
 #### Color diffs