docs: detail how to add or change mappings

Add details on how add or change mappings, letting users know what the
expected value is for a mapping, as it was not clear before.

Closes: GH-91
Signed-off-by: Filip Dutescu <filip.dutescu@gmail.com>

README.md

@@ -198,6 +198,19 @@ to users and mimic the default behaviour of their normal mode equivalents.
 | `<c-u>`  | Undo changes.                |
 | `<c-r>`  | Redo undone changes.         |
 
+To change existing mappings or add new ones, the value needed to be passed in the
+mappings field is a Lua function. Here is an example of how to change `<c-c>`'s
+behaviour from clearing the line to closing the popup:
+
+```lua
+-- other setup code...
+
+['<c-c>'] = function()
+    vim.api.nvim_input('<esc>')
+end
+
+-- other setup code...
+```
 
 ## Media
 

doc/renamer.txt

@@ -204,6 +204,20 @@ described in |renamer.setup|:
                         See `:help CTRL-R`
 
 
+To change existing mappings or add new ones, the value needed to be passed in the
+mappings field is a Lua function. Here is an example of how to change `<c-c>`'s
+behaviour from clearing the line to closing the popup:
+>
+    -- other setup code...
+
+    ['<c-c>'] = function()
+        vim.api.nvim_input('<esc>')
+    end
+
+    -- other setup code...
+<
+
+
 
 ===============================================================================
                                                            *renamer.highlights*