Add Type Casting Convention (#25)

* Add guide on comments

* Add Casting convention

* Delete the runtime cast section

Co-authored-by: Chen Yang <yangche@microsoft.com>

README.md

@@ -15,6 +15,7 @@ The Swift coding conventions guide documents many best practices for writing Swi
 * [Immutability](Immutability.md)
 * [Naming](Naming.md)
 * [Property Observers](PropertyObservers.md)
+* [Type Casting](TypeCasting.md)
 * [Type Inference](TypeInference.md)
 
 # Legal Notices

TypeCasting.md

@@ -0,0 +1,39 @@
+# Type Casting
+
+In general, follow the [official guidelines](https://docs.swift.org/swift-book/LanguageGuide/TypeCasting.html).
+
+## Convention - Static casting
+Use keyword `as` for static casting. Only use static casting when automatic type casting is not available or when the compiler's inferred type does not match the intended type.
+
+## Rationale
+Compiler can infer most types correctly so static casting is not necessary in most cases.
+
+## Examples 
+
+``` swift
+// Bad: unnecessary type casting
+let padding = 42 as CGFloat
+
+// Good: use type annotations instead
+let padding: CGFloat = 42
+```
+
+### Necessary type casting
+
+``` swift
+// setTelemetryID is an Objective-C function that takes an NSNumber as the argument
+// id is of type Int, the casting to NSNumber is not automatic
+button.setTelemetryID(id as NSNumber)
+
+let x = 10
+switch x {
+// Without the type cast, 1...Int.max would be interpreted as Range type
+case 1...Int.max as ClosedInterval:
+    ...
+case Int.min..<0 as HalfOpenInterval:
+    ...
+case 0:
+    ...
+}
+
+```