To install globally, copy prepare-commit-msg to /usr/local/share/git-core/templates/hooks and execute:
chmod +x /usr/local/share/git-core/templates/hooks/prepare-commit-msg
To install per-repository, copy prepare-commit-msg to /path/to/repo/.git/hooks/prepare-commit-msg and mark it as executable.
The branch name will be automatically appended to commit messages using git commit or git commit -m "message".
To include a branch description in your desired branch, execute:
git branch --edit-description
This will open your editor, type a description, save, and exit.
If you'd like to use this commit message hook in an existing repository, after copying the file to the proper location, run git init within the repository for the hook to be initialized.