Expand on_highlights example in README

The examples that were in there previously didn't contain very
constructive information. This should clarify what one is changing and
what else one needs to do to achieve the same.

README.md

@@ -173,22 +173,40 @@ the undercurl:
 
 ```lua
 require("everforest").setup({
-  on_highlights = function (hl, palette)
+  on_highlights = function(hl, palette)
     hl.DiagnosticError = { fg = palette.none, bg = palette.none, sp = palette.red }
     hl.DiagnosticWarn = { fg = palette.none, bg = palette.none, sp = palette.yellow }
     hl.DiagnosticInfo = { fg = palette.none, bg = palette.none, sp = palette.blue }
     hl.DiagnosticHint = { fg = palette.none, bg = palette.none, sp = palette.green }
-  end
+  end,
+})
+```
+
+If you want to tweak or amend an existing highlight group you **need to add the
+colours that aren't changing as well as your new styles**. This is because the
+highlights defined in the `on_highlights` method will _override_ the default
+highlights.
+
+Here's an example of adding a **bold** styling to the `Boolean` highlight group:
+
+```lua
+require("everforest").setup({
+  on_highlights = function(hl, palette)
+    -- The default highlights for Boolean is fg purple, bg none. If we want to
+    -- just add a bold to the existing, we need to have the existing *and* the
+    -- bold style.
+    hl.Boolean = { fg = palette.purple, bg = palette.none, bold = true }
+  end,
 })
 ```
 
 To clear any highlight groups, simply set them to `{}`:
 
 ```lua
 require("everforest").setup({
-  on_highlights = function (hl, palette)
+  on_highlights = function(hl, palette)
     hl.TSDanger = {}
-  end
+  end,
 })
 ```