add description and examples for minLength keyword (#109)

This PR addressed #89

content/2020-12/validation/minLength.markdown

@@ -16,3 +16,49 @@ related:
   - vocabulary: format-annotation
     keyword: format
 ---
+
+The `minLength` keyword is used to specify the minimum length of a string instance. It defines the minimum number of characters that a valid string must have to satisfy the schema.
+
+* Applies only to string data types.
+* Value must be a non-negative integer.
+* String length is counted in characters, not bytes.
+* Validation succeeds if the string length is greater than or equal to the specified `minLength`.
+
+## Examples
+
+{{<schema `Schema requiring minimum string length of 5`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "string",
+  "minLength": 5
+}
+{{</schema>}}
+
+{{<instance-pass `An instance with a string length greater than or equal to 5 is valid`>}}
+"This is a valid string"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance with a string length less than 5 is invalid`>}}
+"foo"
+{{</instance-fail>}}
+
+{{<schema `Schema which allows either a string with at least 3 characters or a numeric value`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": [ "string", "number" ],
+  "minLength": 3
+}
+
+{{</schema>}}
+
+{{<instance-pass `An instance with a string length greater than or equal to 3 is valid`>}}
+"foo"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance with a string length less than 3 is valid`>}}
+"hi"
+{{</instance-fail>}}
+
+{{<instance-pass `An instance with a numeric value is valid`>}}
+55
+{{</instance-pass>}}