feat: docs update (vim-fugitive is optional)

README.md

@@ -11,7 +11,7 @@ highlighting.
 
 - Treesitter syntax highlighting in `:Git` diffs and commit views
 - `:Gdiffsplit` / `:Gvdiffsplit` syntax through diff backgrounds
-- Background-only diff colors for any `&diff` buffer
+- Background-only diff colors for any `&diff` buffer (`:diffthis`, `vimdiff`)
 - Vim syntax fallback for languages without a treesitter parser
 - Hunk header context highlighting (`@@ ... @@ function foo()`)
 - Configurable debouncing, max lines, and diff prefix concealment
@@ -19,7 +19,8 @@ highlighting.
 ## Requirements
 
 - Neovim 0.9.0+
-- [vim-fugitive](https://github.com/tpope/vim-fugitive)
+- [vim-fugitive](https://github.com/tpope/vim-fugitive) (optional, for unified
+  diff syntax highlighting)
 
 ## Installation
 
@@ -39,6 +40,26 @@ Using [lazy.nvim](https://github.com/folke/lazy.nvim):
 :help diffs.nvim
 ```
 
+## Highlight Groups
+
+diffs.nvim defines the following highlight groups. All use `default = true`, so
+colorschemes can override them.
+
+| Group             | Purpose                                            |
+| ----------------- | -------------------------------------------------- |
+| `DiffsAdd`        | Background for `+` lines in fugitive unified diffs |
+| `DiffsDelete`     | Background for `-` lines in fugitive unified diffs |
+| `DiffsAddNr`      | Line number highlight for `+` lines                |
+| `DiffsDeleteNr`   | Line number highlight for `-` lines                |
+| `DiffsDiffAdd`    | Background-only `DiffAdd` for `&diff` windows      |
+| `DiffsDiffDelete` | Background-only `DiffDelete` for `&diff` windows   |
+| `DiffsDiffChange` | Background-only `DiffChange` for `&diff` windows   |
+| `DiffsDiffText`   | Background-only `DiffText` for `&diff` windows     |
+
+By default, these are computed from your colorscheme's `DiffAdd`, `DiffDelete`,
+`DiffChange`, `DiffText`, and `Normal` groups. To customize, define them in your
+colorscheme before diffs.nvim loads, or link them to existing groups.
+
 ## Known Limitations
 
 - Syntax "flashing": diffs.nvim hooks into the `FileType fugitive` event