Add note for automake req for ./autogen.sh step.

Ran into error:

```bash
~/lxc-0.8.0$ ./autogen.sh 
+ test -d autom4te.cache
+ aclocal -I config
./autogen.sh: 6: ./autogen.sh: aclocal: not found
+ exit 1
```

Solved by apt-get install automake. Worth noting as not all cloud linux images come pre-installed with automake.