improve Manual installation section

Author: stevemao

readme.md

@@ -38,15 +38,18 @@ npm install -g diff-so-fancy
 This will install and link the `diff-so-fancy` and `diff-highlight` scripts.
 
 ### Manual installation alternative
+
 If you want, you can choose to install manually:
 
 * Grab the two scripts (`diff-highlight` and `diff-so-fancy`) via either downloading or cloning the repo.
-* Place them in a location that is in your `PATH`.
+* If you download `diff-highlight` from the official git repo, make sure it's executable.
+* Symlink or place them in a location that is in your `PATH`.
 * Set up the git `core.pager` config, as described above.
 
 Note: The `diff-highlight` dependency is an [official git-contrib script](https://github.com/git/git/tree/master/contrib/diff-highlight), duplicated here for convenience. If you prefer less fancy in your diff, you also use diff-highlight [on it's own](https://news.ycombinator.com/item?id=11068436).
 
 ### Install GNU `sed`
+
 You'll need GNU sed. On Mac, it's easy to install with Homebrew.
 ```shell
 brew install gnu-sed