Add more helpful information to the README.

Signed-off-by: Nicolás Ortega Froysa <nicolas@ortegas.org>

README.md

@@ -7,7 +7,7 @@ to boot from a USB depending on just how broken it is).
 
 ## Installation
 
-Dependencies:
+### Dependencies
 
 - Perl 5
 - `File::ReadBackwards` module
@@ -20,6 +20,31 @@ You can install these packages with the following command:
 # pacman -S perl perl-file-readbackwards perl-par-packer
 ```
 
+### Compiling & Installing
+
+The script is compiled and installed using GNU Makefile. Therefore you can use
+`make install` to build and install the script and its man-page as expected.
+They are installed (by default) to `/usr/local`. To change this to a different
+directory simply prepend the `PREFIX=<path>` to your `make install` command.
+
+## Usage
+
+The first concept to understand is that of a transaction. A transaction is
+defined in the pacman logs as package operations done during a single use of the
+command (or so it seems, at least). If you look at the logs
+(`/var/log/pacman.log`) this would be everything between the lines `[ALPM]
+transaction started` and `[ALPM] transaction completed`. You can set how many
+transactions to list/undo by using the `-t` argument.
+
+There are two modes for undoing pacman transactions:
+
+- Interactive (`-i`, default): will show you a numbered list with all the package
+  operations of the selected transactions.
+- Automatic (`-r`): will automatically undo all package operations of the
+  selected transactions.
+
+Look at the man-page (`man pacundo`) for more information.
+
 ## License
 
 This project is licensed under the terms & conditions of the Zlib license.