Add project descriptor to env var tutorials

Signed-off-by: David Freilich <dfreilich@vmware.com>

content/docs/app-developer-guide/environment-variables.md

@@ -6,7 +6,7 @@ summary="Environment variables are a common way to configure various buildpacks
 
 Environment variables are a common way to configure various buildpacks at build-time.
 
-Here are a few ways you can do so.
+Below are a few ways you can do so. All of them will use our [samples][samples] repo for simplicity.
 
 ### Using CLI arguments (`--env`)
 
@@ -16,9 +16,6 @@ The `--env` parameter must be one of the following:
 - `VARIABLE`, where the value of `VARIABLE` will be taken from the local environment
 
 ##### Example:
-
-For this example we will use our [samples][samples] repo for simplicity.
-
 ```bash
 # clone the repo
 git clone https://github.com/buildpacks/samples
@@ -47,17 +44,14 @@ The following environment variables were set and available to buildpacks at buil
 | `FOO`   | `BAR`   | _current environment_      |
 
 
-### Using files (`--env-file`)
+### Using env files (`--env-file`)
 
 The `--env-file` parameter must be a path to a file where each line is one of the following:
 
 - `VARIABLE=VALUE`
 - `VARIABLE`, where the value of `VARIABLE` will be taken from the current environment
 
 ##### Example:
-
-For this example we will use our [samples][samples] repo for simplicity.
-
 ```bash
 # clone the repo
 git clone https://github.com/buildpacks/samples
@@ -91,4 +85,41 @@ The following environment variables were set and available to buildpacks at buil
 
 > **NOTE:** Variables defined using `--env` take precedence over variables defined in `--env-file`.
 
+### Using Project Descriptor (`project.toml`, or `--descriptor`)
+The `--descriptor` parameter must be a path to a file which follows the project.toml [schema][descriptor-schema].
+You can define environment variables in an `env` table in the file, and pass those into the application.
+
+##### Example:
+```bash
+# clone the repo
+git clone https://github.com/buildpacks/samples
+
+# Add an environment variable to the project.toml
+printf "[[build.env]]\nname='HELLO'\nvalue='WORLD'" >> samples/apps/bash-script/project.toml
+
+# build the app
+pack build sample-app \
+    --builder cnbs/sample-builder:bionic \
+    --buildpack  samples/buildpacks/hello-world/ \
+    --buildpack samples/apps/bash-script/bash-script-buildpack/ \
+    --path  samples/apps/bash-script/
+
+# run the app
+docker run sample-app
+```
+
+The following environment variables were set and available to buildpacks at build-time:
+
+| Name    | Value   |  _Source_                  |
+|---------|---------|----------------------------|
+| `HELLO` | `WORLD` | _hard-coded value in file_ |
+
+<p class="spacer"></p>
+
+> **NOTE:** Variables defined using `--env` or `--env-file` take precedence over variables defined in the `project.toml`.
+
+> **NOTE:** `project.toml` can't detect environment variables (so, for instance, if one ran `export FOO=BAR` and added
+>`name=FOO` to the `project.toml`, it wouldn't detect any value set for `FOO`).
+
+[descriptor-schema]: /docs/reference/project-descriptor/
 [samples]: https://github.com/buildpacks/samples