Document OS-level dependencies #24

The README states that this project is compatible with Amazon Linux 2, Alpine, CentOS, Debian, and Ubuntu but only lists an example Dockerfile with dependencies for a Debian variant.

I'm currently about an hour into trying to pip install awslambaric in the official python:3.8-alpine image, which is based on alpine:3.13. I'm still in the process of discovering build dependencies and what Alpine packages provide them.

A listing of the OS-level dependencies for supported OSes would be greatly helpful.

For Alpine, what I've come up with is:

apk add \
      autoconf \
      automake \
      cmake \
      g++ \
      gcc \
      libexecinfo-dev \
      libffi-dev \
      libtool \
      linux-headers \
      make \
      musl-dev \
      openssl-dev \