Add requirements document

This is designed to be a central place to store all requirements

requirements.md

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+# Pelias Sofware requirements
+
+This is the list of all software requirements for Pelias. We highly recommend using our
+[Docker images](https://hub.docker.com/r/pelias/) to avoid having to even attempt to correctly
+install all our dependencies yourself.
+
+## Node.js
+
+Version 6 or newer
+
+Most Pelias code is written in Node.js. Node.js 8 is recommended.
+Node.js 10 is not as well tested with Pelias yet, but should offer notable performance increases and
+may become the recommendation soon.
+
+We will probably drop support for Node.js 6 in the near future, so that we can use the many features
+supported only in version 8 and above.
+
+## Elasticsearch
+
+Version 2.3 or 2.4
+
+The core data storage for Pelias is Elasticsearch. We recommend the latest in the 2.4 release line.
+
+We do not _yet_ support Elasticsearch 5 or 6, but work is [ongoing](https://github.com/pelias/pelias/issues/461)
+
+## SQLite
+
+Version 3.11 or newerr
+
+Some components of Pelias need a relational database, and Elasticsarch does not provide good
+relational support. We use SQLite in these cases since it's simple to manage and quite performant.
+
+## Libpostal
+
+Pelias relies heavily on the [Libpostal](https://github.com/openvenues/libpostal#installation)
+address parser. Libpostal requires about 4GB of disk space to download all the required data.
+
+## Windows Support
+
+Pelias is not well tested on Windows, but we do wish to support it, and will accept patches to fix
+any issues with Windows support.