Add details for running against pipenv projects

README.md

@@ -28,6 +28,7 @@ with support from Google. This is not an official Google or Trail of Bits produc
   * [Dry runs](#dry-runs)
 * [Examples](#examples)
 * [Troubleshooting](#troubleshooting)
+* [Tips and Tricks](#tips-and-tricks)
 * [Security model](#security-model)
 * [Licensing](#licensing)
 * [Contributing](#contributing)
@@ -379,6 +380,27 @@ $ pip-audit --no-deps -r requirements.txt
 $ pip-audit --require-hashes -r requirements.txt
 ```
 
+## Tips and Tricks
+
+### Running against a `pipenv` project
+
+`pipenv` uses both a `Pipfile` and `Pipfile.lock` file to track and freeze dependencies
+instead of a `requirements.txt` file. `pip-audit` cannot process the `Pipfile[.lock]`
+files directly, however, these can be converted to a supported `requirements.txt` file
+that `pip-audit` can run against. Use a Python tool, such as
+[`pipfile-requirements`](https://github.com/frostming/pipfile-requirements), to
+convert your `Pipfile[.lock]` to a `requirements.txt` file and then run
+`pip-audit` against the generated requirements file.
+
+```console
+$ pipenv install --dev pipfile-requirements
+$ pipenv run pipfile2req Pipfile.lock > requirements.txt
+$ pipenv run pip-audit -r requirements.txt
+```
+
+The `requirements.txt` file needs to be kept up to date with `pipenv` when running
+`pip-audit` locally like this.
+
 ## Security Model
 
 This section exists to describe the security assumptions you **can** and **must not**