Add debian package build information in readme

Add prerequisites for building a debian package.

README.md

@@ -19,6 +19,13 @@ cargo b --release
 ```
 
 ### Build the `mqrs` Debian package
+For that [`cargo-deb`](https://github.com/mmstick/cargo-deb) is
+required, which can be installed with:
+```sh
+cargo install cargo-deb
+```
+
+A Debian package can be built with:
 ```sh
 cargo deb
 ```
@@ -42,8 +49,7 @@ cargo b
 ```
 
 or you can copy the produced binary somewhere else or link to them
-from `target/{debug,release}/mqrs` depending on which
-one you built.
+from `target/{debug,release}/mqrs` depending on which one you built.
 
 ## Using `mqrs`
 Depending on which backend you want to use there are different subsets