Add `libpq-dev` to instructions

Fixes #238

CONTRIBUTING.md

@@ -99,6 +99,8 @@ documentation section: [Translation](https://docs.djangoproject.com/en/3.0/topic
 - Python 3.6+, pip, pipenv
 - NodeJS 14.x and NPM 7.x
 - Gulp
+- Possibly `libpq-dev`
+  - This is necessary if `psycopg2` can't find an appropriate prebuild binary.
 
 ### Installation
 
@@ -108,6 +110,8 @@ documentation section: [Translation](https://docs.djangoproject.com/en/3.0/topic
 
        pipenv install --three
 
+   - If this fails, install `libpq-dev` (e.g. `sudo apt install libpq-dev`) and try again.
+
 1. Install Gulp CLI
 
        sudo npm install -g gulp-cli

README.md

@@ -161,7 +161,7 @@ Python 3.6+, nginx, uwsgi and sqlite and should be sufficient for a few users
 
 1. Install system packages
 
-        sudo apt-get install python3 python3-pip nginx uwsgi uwsgi-plugin-python3 git libopenjp2-7-dev
+        sudo apt-get install python3 python3-pip nginx uwsgi uwsgi-plugin-python3 git libopenjp2-7-dev libpq-dev
 
 1. Default python3 to python for this session