DOC: Add a minimal description of a NiPreps project

Add a minimal description of a data processing pipeline project under
the `NiPreps` convention.

Author: jhlegarreta

README.md

@@ -0,0 +1,50 @@
+# NiPreps Project Template
+
+## Overview
+
+This repository contains a template to be used as a starting point for a new *NiPrep* processing pipeline. 
+
+A *NiPreps* project named `{{project_name}}` (e.g. `MyPrep`) will typically have the following structure:
+
+```
+{{project_slug}}/
+├── .github/
+│   └── ...
+├── .maint/
+│   └──  ...
+├── docs/
+│   └── ...
+├── tests/
+│   └──  ...
+├── {{project_slug}}/
+│   ├── cli/
+│   │   ├── __init__.py
+│   │   ├── parser.py
+│   │   ├── run.py
+│   │   ├── version.py
+│   │   └── workflow.py
+│   ├── data/
+│   │   ├── __init__.py
+│   │   └── ...
+│   ├── interfaces/
+│   │   ├── __init__.py
+│   │   └── ...
+│   ├── utils/
+│   │   ├── __init__.py
+│   │   └── bids.py
+│   ├── workflows/
+│   │   └── base.py
+│   ├── __init__.py
+│   ├── __main__.py
+│   ├── config.py
+│   └── conftest.py
+├── ...
+├── Dockerfile
+├── ...
+├── pyproject.toml
+├── README.md
+└── tox.ini
+```
+
+where `{{project_slug}}` corresponds to the all lowercase name of the project to be used as the root directory to host
+the relevant processing source code (e.g. `myprep`).