Add build options examples

This adds an example of how to practically use `build-option`
with pillow for Python

Signed-off-by: Ivana Yovcheva (VMware) <[email protected]>

Author: ivanayov

docs/cli/build.md

@@ -101,4 +101,61 @@ Remeber to add any `ARG` values to the template's Dockerfile:
  ARG ARGNAME2
  ```
 
- For more information about passing build arguments to Docker, please visit the [Docker documentation](https://docs.docker.com/engine/reference/commandline/build/)
+ For more information about passing build arguments to Docker, please visit the [Docker documentation](https://docs.docker.com/engine/reference/commandline/build/)
+
+## 2.1 Build options examples
+
+Let's see some practical examples with the `build-option` flag.
+
+* Use [Pillow](https://pillow.readthedocs.io/en/5.2.x/) for image processing in your Python function
+
+Create function with
+
+```
+$ faas-cli new faas_black_and_white --lang python3 --prefix <your-docker-namespace>
+```
+
+Add `pillow` to `requirements.txt` .
+
+Copy some resource images to `faas_black_and_white/`.
+
+Edit `handler.py`:
+
+```python
+import os
+from PIL import Image
+
+def handle(req):
+    img = Image.open("function/" + req)
+
+    blackAndWhite = img.convert('1')
+
+    blackAndWhite.save("function/blackAndWhite.jpeg")
+
+    with open("function/blackAndWhite.jpeg", 'rb') as pic:
+        res = pic.read()
+
+    os.write(1, res)
+    return res
+```
+
+What the code does is to open an image, available in resources and convert it to black and white.
+
+Now build the function with build options:
+```
+faas build -f faas_black_and_white.yml --build-option dev --build-option pillow
+```
+
+Push and deploy:
+```
+faas push -f faas_black_and_white.yml && faas deploy -f faas_black_and_white.yml
+```
+
+Test the function with:
+
+```bash
+echo "image-name.jpg" | faas invoke pillow-func > blackAndWhite.jpg
+```
+
+Or in your web browser by opening http://127.0.0.1:8080/function/pillow-func
+