Derive JsonSchema_repr (#76)

docs/_includes/examples/enum_repr.rs

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+use schemars::{schema_for, JsonSchema_repr};
+
+#[derive(JsonSchema_repr)]
+#[repr(u8)]
+enum SmallPrime {
+    Two = 2,
+    Three = 3,
+    Five = 5,
+    Seven = 7,
+}
+fn main() {
+    let schema = schema_for!(SmallPrime);
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}

docs/_includes/examples/enum_repr.schema.json

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+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "SmallPrime",
+  "type": "integer",
+  "enum": [
+    2,
+    3,
+    5,
+    7
+  ]
+}

docs/examples/8-enum_repr.md

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+---
+layout: default
+title: Serialize enum as number (serde_repr)
+parent: Examples
+nav_order: 8
+summary: >-
+  Generating a schema for with a C-like enum compatible with serde_repr.
+---
+
+# Deriving JsonSchema with Fields Using Custom Serialization
+
+If you use the `#[repr(...)]` attribute on an enum to give it a C-like representation, then you may also want to use the [serde_repr](https://github.com/dtolnay/serde-repr) crate to serialize the enum values as numbers. In this case, you should use the corresponding `JsonSchema_repr` derive to ensure the schema for your type reflects how serde formats your type.
+
+{% include example.md name="enum_repr" %}