Add dev installation instructions to contributing page

CONTRIBUTING.md

@@ -4,6 +4,7 @@ This project welcomes contributions and suggestions.
 
  - [Code of Conduct](#coc)
  - [Issues and Bugs](#issue)
+ - [Development Installation](#dev)
  - [Feature Requests](#feature)
  - [Submission Guidelines](#submit)
 
@@ -13,6 +14,19 @@ Help us keep this project open and inclusive. Please read and follow our [Code o
 ## <a name="issue"></a> Found an Issue?
 If you find a bug in the source code or a mistake in the documentation, you can help us by [submitting an issue](#submit-issue) to the GitHub Repository. Even better, you can [submit a Pull Request](#submit-pr) with a fix.
 
+### <a name="dev"></a> Development Installation
+
+This package uses [Poetry](https://python-poetry.org/) to manage dependencies.
+After following Poetry's own installation instructions, and with an appropriate
+Python version set up, you can install PASCal with all required development
+dependencies using:
+
+```shell
+git clone [email protected]:MJCliffe/PASCal
+cd PASCal
+poetry install --all-extras
+```
+
 ## <a name="feature"></a> Want a Feature?
 You can *request* a new feature by [submitting an issue](#submit-issue) to the GitHub Repository. If you would like to *implement* a new feature, please submit an issue with a proposal for your work first, to be sure that we can use it.
 
@@ -37,6 +51,7 @@ features, by not reporting duplicate issues.  Providing the following informatio
 
 You can file new issues by providing the above information at the corresponding repository's issues link: https://github.com/MJCliffe/PASCal/issues/new].
 
+
 ### <a name="submit-pr"></a> Submitting a Pull Request (PR)
 Before you submit your Pull Request (PR) consider the following guidelines: