Add DEVELOP.md dev environment setup instructions

DEVELOP.md

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+pySMART
+===========
+
+Copyright (C) 2021-2023 [Rafael Leira](https://github.com/ralequi)\
+Copyright (C) 2021 [Truenas team](https://www.truenas.com/)\
+Copyright (C) 2015 Marc Herndon
+
+Development
+-----------
+
+To run the tests, or to contribute to pySMART, you will have to set up a
+development environment.
+
+Our development environment will consist of a virtual environment, with the
+installation and development dependencies installed.
+
+See the [Python documentation] for more on creating and using virtual
+environment.
+
+This document will assume a Linux environment.
+
+The steps are:
+
+1. Clone this repository, go there
+1. Create a virtualenv and activate it
+1. Install the dependencies
+1. Run the tests
+
+- Go where you want to clone the repository, clone it, go there:
+
+    ```bash
+    $ cd <wherever you want to clone the repository>
+    $ git clone https://github.com/truenas/py-SMART
+
+    Cloning into 'py-SMART'...
+    remote: Enumerating objects: 2243, done.
+    remote: Counting objects: 100% (839/839), done.
+    remote: Compressing objects: 100% (362/362), done.
+    remote: Total 2243 (delta 544), reused 743 (delta 463), pack-reused 1404
+    Receiving objects: 100% (2243/2243), 674.95 KiB | 2.38 MiB/s, done.
+    Resolving deltas: 100% (1518/1518), done.
+
+    $ cd py-SMART
+    ```
+
+- Create a virtualenv and activate it
+
+    ```bash
+    $ python -m venv .venv
+    $ source .venv/bin/activate
+    (.venv) $
+    ```
+
+    We use `.venv` for the directory name of the virtualenv by convention.
+    Any other legal directory name is fine; we'll assume `.venv`.
+
+- Update pip and setuptools
+
+    ```bash
+    $(.venv) $ python -m pip install --upgrade pip setuptools
+
+    Requirement already satisfied: pip in <...>/.venv/lib/python3.12/site-packages (23.1)
+    Collecting pip
+    Using cached pip-24.0-py3-none-any.whl (2.1 MB)
+    Requirement already satisfied: setuptools in <...>/.venv/lib/python3.12/site-packages (68.0.0)
+    Collecting setuptools
+    Downloading setuptools-69.5.1-py3-none-any.whl (894 kB)
+        ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 894.6/894.6 kB 6.1 MB/s eta 0:00:00
+    Installing collected packages: setuptools, pip
+    Attempting uninstall: setuptools
+        Found existing installation: setuptools 68.0.0
+        Uninstalling setuptools-68.0.0:
+        Successfully uninstalled setuptools-68.0.0
+    Attempting uninstall: pip
+        Found existing installation: pip 23.1
+        Uninstalling pip-23.1:
+        Successfully uninstalled pip-23.1
+    Successfully installed pip-24.0 setuptools-69.5.1
+    ```
+
+- Install the package including development dependencies
+
+    ```bash
+    $(.venv) $ python -m pip install --editable .[dev]
+    ```
+
+- Run the tests
+
+    ```bash
+    $(.venv) $ pytest
+    =============== test session starts =====================================
+    platform linux -- Python 3.12.2, pytest-8.1.1, pluggy-1.5.0
+    rootdir: <...>/py-SMART
+    configfile: pyproject.toml
+    plugins: cov-5.0.0
+    collected 184 items
+    < test file names, lots and lots of dots>
+    =============== 184 passed in 0.99s =====================================
+    ```
+
+**That's it -- you're ready to start developing on pySMART!**
+
+[Python documentation]: https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/#create-and-use-virtual-environments