docs: Add commit example using vim.ui.input() (#425)

Co-authored-by: Sindre T. Strøm <[email protected]>

Author: LintaoAmons

USAGE.md

@@ -204,6 +204,78 @@ keymaps = {
 }
 ```
 
+### Use `vim.ui.input()`
+
+This example shows how to use the Neovim builtin `vim.ui.input()` to create a
+simple input prompt, and create a new commit with the user's given message.
+
+![vim.ui.input() demo](https://user-images.githubusercontent.com/2786478/274230878-661a30d5-cc1d-4a34-b196-c4adf9e9f8c5.gif)
+
+For Neovim ≥ v0.10:
+
+```lua
+keymaps = {
+  file_panel = {
+    {
+      "n", "cc",
+      function()
+        vim.ui.input({ prompt = "Commit message: " }, function(msg)
+          if not msg then return end
+          local results = vim.system({ "git", "commit", "-m", msg }, { text = true }):wait()
+
+          if results.code ~= 0 then
+            vim.notify(
+              "Commit failed with the message: \n"
+                .. vim.trim(results.stdout .. "\n" .. results.stderr),
+              vim.log.levels.ERROR,
+              { title = "Commit" }
+            )
+          else
+            vim.notify(results.stdout, vim.log.levels.INFO, { title = "Commit" })
+          end
+        end)
+      end,
+    },
+  },
+}
+```
+
+<details>
+  <summary><b>For Neovim ≤ v0.10, use this function in place of <code>vim.system()</code></b></summary>
+
+```lua
+--- @class system.Results
+--- @field code integer
+--- @field stdout string
+--- @field stderr string
+
+--- @param cmd string|string[]
+--- @return system.Results
+local function system(cmd)
+  local results = {}
+
+  local function callback(_, data, event)
+    if event == "exit" then results.code = data
+    elseif event == "stdout" or event == "stderr" then
+      results[event] = table.concat(data, "\n")
+    end
+  end
+
+  vim.fn.jobwait({
+    vim.fn.jobstart(cmd, {
+      on_exit = callback,
+      on_stdout = callback,
+      on_stderr = callback,
+      stdout_buffered = true,
+      stderr_buffered = true,
+    })
+  })
+
+  return results
+end
+```
+</details>
+
 ### Use `:!cmd`
 
 If you only ever write simple commit messages you could make use of `:h !cmd`: