Add new variable: g:codeium_filetypes_disabled_by_default.

Enables finer control of when to enable codeium.

README.md

@@ -116,15 +116,27 @@ let g:codeium_filetypes = {
 
 Codeium is enabled by default for most filetypes.
 
-You can also _disable_ codeium by default with the `g:codeium_enabled`
-variable:
+You can also _disable_ codeium by default with the `g:codeium_enabled` variable,
+and enable it manually per buffer by running `:CodeiumEnable`:
 
 ```vim
 let g:codeium_enabled = v:false
 ```
 
-Instead, if you would like to just disable the automatic triggering of
-completions:
+Or you can disable codeium for _all filetypes_ with the `g:codeium_filetypes_disabled_by_default` variable,
+and use the `g:codeium_filetypes` variable to selectively enable codeium for specified filetypes:
+
+```vim
+" let g:codeium_enabled = v:true
+let g:codeium_filetypes_disabled_by_default = v:true
+
+let g:codeium_filetypes = {
+    \ "rust": v:true,
+    \ "typescript": v:true,
+    \ }
+```
+
+If you would like to just disable the automatic triggering of completions:
 
 ```vim
 let g:codeium_manual = v:true