Change asciicast in README, make usage significantly more thorough.

Author: quodlibetor

README.md

@@ -2,7 +2,29 @@
 
 Quickly fix up an old commit using your currently-staged changes.
 
-[![asciicast](./static/asciicast.png)](https://asciinema.org/a/bLZ1eFaDTKKvVMtPzgTUgYNVG)
+[![asciicast](./static/asciicast.png)](https://asciinema.org/a/SYKj4ztmMJ52cSmGxSF9hHjtl?autoplay=1&t=3)
+
+## Usage
+
+After installation, just run `git fixup` or `git squash` to perform the related
+actions.
+
+Running `git fixup` will check if you have any staged changes (if not it will
+prompt you to stage all changes) and then present you with a list of commits
+from your current work point (HEAD) to HEAD's upstream. For example, if you are
+on `master` and its is `origin/master`, `git fixup` will show all commits
+between `master` and `origin/master`. In general this is just what you want,
+since you probably shouldn't be editing commits that other people are working
+off of.
+
+After you select the commit to edit, `git fixup` will apply your staged changes
+to that commit without any further prompting or work from you.
+
+`git-squash` is just a symlink to `git-fixup` installed by brew, but if you
+invoke it (either as `git-squash` or `git squash`) it will behave the same,
+asking you which change to amend, but after you have selected the commit to git
+will give you a chance to edit the commit message before changing the tree at
+that point.
 
 ## Installation
 
@@ -18,10 +40,6 @@ build, and then copy the binary into your bin dir:
     cargo build --release
     cp target/release/git-fixup /usr/local/bin/git-fixup
 
-## Usage
-
-After installation, just run `git fixup` or `git squash` to perform the related actions.
-
 ## License
 
 git-fixup is licensed under either of