Recommend to use ReadonlyArray (#18429)

README.md

@@ -133,6 +133,7 @@ For a good example package, see [base64-js](https://github.com/DefinitelyTyped/D
 
 * First, follow advice from the [handbook](http://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html).
 * Formatting: Either use all tabs, or always use 4 spaces.
+* `function sum(nums: number[]): number`: Use `ReadonlyArray` if a function does not write to its parameters.
 * `interface Foo { new(): Foo; }`:
     This defines a type of objects that are new-able. You probably want `declare class Foo { constructor(); }`.
 * `const Class: { new(): IClass; }`: