Generate schema from any serializable value (#75)

Implement schema_for_value!(...) macro

docs/_includes/examples/from_value.rs

@@ -0,0 +1,24 @@
+use schemars::schema_for_value;
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct MyStruct {
+    pub my_int: i32,
+    pub my_bool: bool,
+    pub my_nullable_enum: Option<MyEnum>,
+}
+
+#[derive(Serialize)]
+pub enum MyEnum {
+    StringNewType(String),
+    StructVariant { floats: Vec<f32> },
+}
+
+fn main() {
+    let schema = schema_for_value!(MyStruct {
+        my_int: 123,
+        my_bool: true,
+        my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
+    });
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}