docs(task): doc for lua task provider

docs/documentation.md

@@ -2346,6 +2346,43 @@ endfunction
 call SpaceVim#plugins#tasks#reg_provider(function('s:make_tasks'))
 ```
 
+The provider also can be implemented in lua, for example:
+
+```lua
+local task = require('spacevim.plugin.tasks')
+
+local function make_tasks()
+  if vim.fn.filereadable('Makefile') then
+    local subcmds = {}
+    local conf = {}
+    for _, v in ipairs(vim.fn.readfile('Makefile', '')) do
+      if vim.startwith(v, '.PHONY') then
+        table.insert(subcmds, v)
+      end
+    end
+    for _, subcmd in ipairs(subcmds) do
+      local comamnds = vim.fn.split(subcmd)
+      table.remove(commands, 1)
+      for _, cmd in ipairs(commands) do
+        conf = vim.tbl_extend('forces', conf, {
+          [cmd] = {
+            command = 'make',
+            args = {cmd}
+            isDetected = true,
+            detectedName = 'make:'
+          }
+        })
+      end
+    end
+    return conf
+  else
+    return {}
+  end
+end
+
+task.reg_provider(make_tasks)
+```
+
 With the above configuration, you will see the following tasks in the SpaceVim repo:
 
 ![task-make](https://img.spacevim.org/75105016-084cac80-564b-11ea-9fe6-75d86a0dbb9b.png)