README: Add install instructions for Alpine Linux

[mil]

Adds install instructions for Alpine Linux

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[davidgamez]

Hi @mil, thanks for your contribution(s). We don't have Alpine Linux in-house, and we have limited skills in Linux, so I'm sorry if I'm missing something obvious that a more expert user can catch.
I followed the steps in docker and also in AWS with a community-provided Alpine distributions. In both cases, I got the following error:

ERROR: unable to select packages:
  libspatialindex-dev (no such package):
    required by: world[libspatialindex-dev]

In removed the libspatialindex-dev and I was able to install the requirements but first I needed to install some additional packages that were not in the fresh Alpine distribution: g++ gcc musl-dev libffi-dev. This is the list of packages that I installed before being able to install libs from requirements.txt:

apk add --no-cache gdal-dev g++ gcc musl-dev proj proj-dev proj-util python3-dev libffi-dev

I just want to make sure that an Alpine user gets these instructions and knows how to fix potential issues. Does the libs that I installed make sense to mention them in the readme? Do we need to mention particular requirements for the Alpine distribution? Any other thoughts?
Thanks again for your precious time and effort.