CONTRIBUTING: Add automake command

Otherwise there is no configure script to execute.

Author: corubba

CONTRIBUTING.md

@@ -63,7 +63,7 @@ and ask!
 
 6) Configure and build
    ```
-   ./configure && make -s && make check
+   ./autogen.sh && ./configure && make -s && make check
    ```
 
 7) Add/update tests