docs(mail): add doc for mail layer

docs/layers/mail.md

@@ -0,0 +1,37 @@
+---
+title: "SpaceVim mail layer"
+description: "mail layer provides basic mail client for SpaceVim."
+---
+
+# [Available Layers](../) >> mail
+
+<!-- vim-markdown-toc GFM -->
+
+- [Description](#description)
+- [Layer options](#layer-options)
+- [Key Mappings](#key-mappings)
+
+<!-- vim-markdown-toc -->
+
+## Description
+
+The `mail` layer provides basic function to connected to mail server.
+To use this configuration layer, add the following snippet to your custom configuration file.
+
+```toml
+[[layers]]
+  name = "mail"
+```
+
+## Layer options
+
+1. `imap_host`: set the imap server host
+2. `imap_port`: set the imap server port
+3. `imap_login`: set the login of imap server
+4. `imap_password`: set the password of imap server
+
+## Key Mappings
+
+| Key Bingding | Description      |
+| ------------ | ---------------- |
+| `SPC a m`    | open mail client |