Add "Style Options" section for bulma CSS

This project uses bulma CSS, but this fact is not obvious without reading the code. This commit adds explicit pointers on how to use the `tagstyle` field.

README.md

@@ -127,3 +127,14 @@ If you choose to fetch message information from an endpoint, the output format s
 
 `null` value or missing keys will be ignored and value from the `config.yml` will be used if available.
 Empty values (either in `config.yml` or the endpoint data) will hide the element (ex: set `"title": ""` to hide the title bar).
+
+### Style Options
+
+Homer uses [bulma CSS](https://bulma.io/), which provides a [modifiers syntax](https://bulma.io/documentation/modifiers/syntax/). You'll notice in the config there is a `tagstyle` option. It can be set to any of the bulma modifiers. You'll probably want to use one of these 4 main colors:
+
+- `is-info` (blue)
+- `is-success` (green)
+- `is-warning` (yellow)
+- `is-danger` (red)
+
+You can read the [bulma modifiers page](https://bulma.io/documentation/modifiers/syntax/) for other options regarding size, style, or state.