add documentation for z formatting

I didn't see anything in the readme about contributions, but I'm hoping this is acceptable - I'm not 100% sure it fits with "parsing" - I haven't tried that side of the feature, but it does work for formatting per: iamkun/dayjs#1069 

Thanks for all your work on this package!

docs/parse/string-format.md

@@ -30,7 +30,7 @@ If you don't know the exact format of an input string, but know it could be one
 dayjs("12-25-2001", ["YYYY", "YYYY-MM-DD"], 'es', true);
 ```
 
-### List of all available parsing tokens
+### List of available parsing tokens in the CustomParseFormat plugin 
 
 | Input  | Example          | Description                       |
 | ------ | ---------------- | --------------------------------- |
@@ -62,4 +62,16 @@ dayjs("12-25-2001", ["YYYY", "YYYY-MM-DD"], 'es', true);
 | `x`    | 1410715640579    | Unix ms timestamp                 |
 
 
+For additional formatting, version 1.9+ added timezone abbreviations using the advancedFormat plugin
+```
+var advanced = required("dayjs/plugin/advancedFormat")
+dayjs.extend(advanced)
+```
+
+### List of all available parsing tokens with advancedFormat plugin
+
+| Input  | Example          | Description                       |
+| ------ | ---------------- | --------------------------------- |
+| `z`    | 01               | Timezone Abbreviation             |
+