unmarked dependency on libpcre3 might break application

[scones]

Hello MtHaml maintainers,

This library does require libpcre3 to be installed on the system.
This should be documented somewhere in this repository (ie. the README).

I had the issue, that the libpcre3 version on my container was 2:8.35-3.3+deb8u4, which crashed as soon as i used MtHaml to render .haml files.
I am aware, that this is not a problem of MtHaml itself, although lacking documentation on a dependency, thus increasing debug time, is a problem with the library.

in the hopes all dependencies will be documented properly,
best regards,
scones