Add wish_append and wish_remaining_chars to Plugins.md

Plugins.md

@@ -15,7 +15,7 @@
 + [Username](plugins/username)
 + [vcs](plugins/vcs)
 
-# How to create new plugin
+# How to create a new plugin
 
 > Note: Replace `pluginname` with actual plugin name
 
@@ -26,4 +26,27 @@ Plugins need to be defined in a file named `pluginname.sh`. Plugins are sourced
 + `wish_pluginname_end` - This function should return 0 or 1 if it allows/disallows powerline before it respectively.
 + `wish_pluginname_main` - This function is called every time a prompt is generated. Ensure to run `wish_append bg_color fg_color text` within this function. Although the plugin can override the theme while running `wish_append`, it is not recommended.
 
+Plugins have two functions available:
+
++ `wish_append`: Appends text to prompt. Plugins need to call this to add text
+  to prompt in their `main` function. They can call it multiple times (or never)
+  too.
++ `wish_remaining_chars`: Returns remaining characters in current line.
+
+## `wish_append`
+
++ Usage: `wish_append bg fg text`
++ Parameters:
+  - `fg`, `bg`: -1 to reset, 0-255 for terminal color codes. 6 digit hexadecimal
+    value for true color.
+  - `text`: Text of the plugin
++ Return value: `None`
+
+## `wish_remaining_chars`
+
++ Usage: `wish_remaining_chars`
++ Parameters: `None`
++ Return Value: Capture stdout to get the remaining characters available in the
+line.
+
 Check [dummy plugin](https://gitlab.com/ceda_ei/wish/blob/master/plugins/dummy.sh).