📝 docs(gitbook): add built-in validator section

Author: kazupon

gitbook/en/SUMMARY.md

@@ -8,3 +8,4 @@
 - [Getting Started](started.md)
 - [Validation With Component](validate.md)
 - [Validation Results](results.md)
+- [Built-in Validators](validators.md)

gitbook/en/validators.md

@@ -0,0 +1,12 @@
+# Built-in validators
+
+You can use any of the built-in validators listed below:
+
+- `required`: whether the value has been specified
+- `pattern`: whether the value suffice the pattern of the regular expression
+- `minlength`: whether the length of the specified value is more than or equal to a minimum length
+- `maxlength`: whether the length of the specified value is less than or equal to a maximum length
+- `min`: whether the specified numerical value is more than or equal to a minimum
+- `max`: whether the specified numerical value is less than or equal to a maximum
+
+See more about [API section](/api.html#buildin-validators).