doc: Add declaration of method annotations

manual/annotations.md

@@ -143,6 +143,25 @@ This annotation tells instance variable of instance.
 * `@type` `instance` `ivar` *ivar* `:` *type*
 * `@type` `module` `ivar` *ivar* `:` *type*
 
+## Method Annotations
+
+Method annotations are about hinting behavior of the method.
+
+This kind of annotations should be written in the method context using annotation notation (ex. `%a{...}`) in the RBS file.
+
+Supported annotations are:
+
+* `pure`: This method is a pure method.  It means this method do not have side effects, and its return type will not be changed during the conditional block.
+
+### Example
+
+```rbs
+class Foo
+  %a{pure}
+  def foo: () -> String
+end
+```
+
 ## Type assertion
 
 Type assertion allows declaring type of an expression inline, without introducing new local variable with variable type annotation.