Document OS-level dependencies

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 \
```