docs: add configuration and describe features

Author: seflue

README.md

@@ -1,6 +1,7 @@
 # telescope-orgmode.nvim
 
-Integration for [orgmode](https://github.com/nvim-orgmode/orgmode) with
+Provides integration for [orgmode](https://github.com/nvim-orgmode/orgmode) and
+[org-roam.nvim](https://github.com/chipsenkbeil/org-roam.nvim) with
 [telescope.nvim](https://github.com/nvim-telescope/telescope.nvim).
 
 ## Demo
@@ -69,3 +70,47 @@ require('telescope').extensions.orgmode.search_headings
 require('telescope').extensions.orgmode.refile_heading
 require('telescope').extensions.orgmode.insert_link
 ```
+
+## Toggle between headline and org file search
+
+By pressing `<C-Space>` the picker state can be toggled between two modes.
+Every mode is available in every function.
+
+### Search headlines
+
+This is the first and default mode. It shows all the headlines, initially
+sorted by most recently changed org file. The level of headlines can be
+[configured](#Configuration).
+
+### Search org files
+
+This is the second mode, which shows only org files. If the org file has a
+title, it is shown (and used for filtering) instead of the filename. This is
+particular useful in connection with
+[org-roam.nvim](https://github.com/chipsenkbeil/org-roam.nvim) to fuzzy search
+for roam nodes.
+
+## Configuration
+
+You can limit the maximum headline level included in the search. `nil` means
+unlimited level, `0` means only search for whole org files. The later is
+equivalent with [org file mode](#search-org-files)
+
+To enable the configuration for all commands, you pass the option to the setup
+function of telescope:
+
+```lua
+require('telescope').setup({
+    extensions = {
+        orgmode = {
+            max_depth = 3
+        }
+    }
+})
+```
+
+For a particular command you can pass it directly in your key mapping to the function:
+
+```lua
+require('telescope').extension.orgmode.search_headings({ max_depth = 3 })
+```