Add nix config files

INSTALL-NIX.md

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+# Preliminaries for using nix
+
+For a dev environment with nix package manager a postgres install on the base system is required, this guide does not cover a postgresql server running with nix-shell.
+
+## Set up PostgreSQL
+
+```
+sudo -u postgres createdb lamassu
+sudo -u postgres psql postgres
+```
+
+In ``psql``, run the following and set password to ``postgres123``:
+
+```
+\password postgres
+ctrl-d
+```
+
+## Starting up environment
+
+shell.nix script provided, all you need to do to setup the environment is to run `nix-shell` on the folder. 
+
+# Installation
+
+## Install node modules
+
+Make sure you're running NodeJS 8.3 or higher. Ignore any warnings.
+
+```
+npm install
+```
+
+## Generate certificates
+
+```
+bash bin/cert-gen.sh
+```
+
+Notes: 
+  - This will create a ``.lamassu`` directory in your home directory.
+  - The script uses the current openssl LTS version (1.0.2g) and will not work on v1.1.
+  
+## Set up database
+
+Important: lamassu-migrate currently gripes about a QueryResultError. Ignore this, it works anyway. Also, ignore Debug lines from lamassu-apply-defaults.
+
+```
+node bin/lamassu-migrate
+node bin/lamassu-apply-defaults
+```
+
+## Register admin user
+
+You'll use this generated URL in the brower in moment.
+
+```
+node bin/lamassu-register admin
+```
+
+## Run lamassu-admin-server
+
+In first terminal window:
+
+```
+node bin/lamassu-admin-server --dev
+```
+
+## Complete configuration
+
+Paste the URL from lamassu-register exactly as output, into a browser (chrome or firefox).
+
+**Important**: the host must be localhost. Tell your browser to trust the certificate even though it's not signed by a recognized CA.
+
+Go to all the required, unconfigured red fields and choose some values. Choose mock services whenever available.
+
+## Run lamassu-server
+
+In second terminal window:
+
+```
+node bin/lamassu-server --mockSms
+```
+
+## Add a lamassu-machine
+
+Click on ``+ Add Machine`` in the sidebar. Type in a name for your machine and click **Pair**. Open up development tools to show the JavaScript console and copy the totem. You will use this to run lamassu-machine. This pairing totem expires after an hour.
+
+Now continue with lamassu-machine instructions from the ``INSTALL.md`` file in lamassu-machine.

shell.nix

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+with import <nixpkgs> {};
+
+stdenv.mkDerivation {
+    name = "node";
+        buildInputs = [
+        nodejs-8_x
+        python2Full
+        openssl_1_0_2
+        postgresql_9_6
+    ];
+    shellHook = ''
+        export PATH="$PWD/node_modules/.bin/:$PATH"
+    '';
+}