docs(readme): Add hint about prettier ignore (#41593)

This is an aftermath of problem discussed here:
https://github.com/DefinitelyTyped/DefinitelyTyped/pull/41567#discussion_r365882974

Thanks!

README.md

@@ -182,7 +182,11 @@ For a good example package, see [base64-js](https://github.com/DefinitelyTyped/D
 #### Common mistakes
 
 * First, follow advice from the [handbook](http://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html).
-* Formatting: Use 4 spaces. Prettier is set up on this repo, so you can run `npm run prettier -- --write path/to/package`.
+* Formatting: Use 4 spaces. Prettier is set up on this repo, so you can run `npm run prettier -- --write path/to/package`. [When using assertions](https://github.com/SamVerschueren/tsd#assertions), add `// prettier-ignore` exclusion to mark line(s) of code as excluded from formatting:
+    ```tsx
+    // prettier-ignore
+    const incompleteThemeColorModes: Theme = { colors: { modes: { papaya: { // $ExpectError
+    ```
 * `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(); }`.