Add numeric-constraints annotation docs

Author: earlbread

docs/annotation.md

@@ -129,3 +129,32 @@ class FileNotFound(FileError):
 class FileNotReadable(FileError):
     ...
 ~~~~~~~~
+
+### `@numeric-constraints`                                {#numeric-constraints}
+
+`@numeric-constraints` annotation constrain the range of the input value.
+Currently, available annotation arguments are below:
+
+`min`: Minimum input value; inclusive
+
+`max`: Maximum input value; inclusive
+
+For example, the following first Nirum code is compiled to the second Python
+code:
+
+~~~~~~~~ nirum
+@numeric-constraints(min=1, max=12)
+unboxed month (int32);
+~~~~~~~~
+
+~~~~~~~~ python
+class Month(object):
+    ...
+    def __init__(self, value: '__builtin__.int') -> None:
+        ...
+        if not (value <= (12)):
+            raise ValueError("value is greater than 12")
+        if not (value >= (1)):
+            raise ValueError("value is less than 1")
+        ...
+ ~~~~~~~~