docs: Add missing build dependency.

[tillnr]

Running ./bootstrap after cloning libyamlfor the first time will lead to to the following output and failure to run the bootstrapping step successfully, even though all documented dependencies are installed:

autoreconf: export WARNINGS=
autoreconf: Entering directory '.'
autoreconf: configure.ac: no obvious need to run autopoint
autoreconf: running: aclocal --force 
Can't exec "aclocal": No such file or directory at /usr/share/autoconf/Autom4te/FileUtils.pm line 297.
autoreconf: error: failed to run aclocal: No such file or directory

aclocal is part of automake and should therefore be mentioned in the dependencies of building after a fresh clone in addition to autoconf.

[perlpunk]

Which distribution are you using? On openSUSE Leap 16.0 automake is a dependency of libtool.
For me OTOH gcc is missing and should maybe be mentioned.

[tillnr]

I am using Arch Linux. Maybe a note like "Depending on your distribution you might need to install automake seperately" would suffice?

[tillnr]

I would be happy to compile a list of common distributions and check if building libyaml on a fresh install is missing dependencies or not.
Is this something that would be helpful?