message: Add info to readme about string enum support
diff: @@ -176,6 +176,7 @@ uniqueId?: string;
  ```
  - `enum`: The enum option accepts a string array. The class property which gets this decorator should have an enum-like type which values are from the provided string array. The way how the enum is created is delegated to the developer, Typegoose needs a string array which hold the enum values, and a TypeScript type which tells the possible values of the enum.
  + However, if you use TS 2.4+, you can use string enum as well.
  ```typescript
  // Enum-like type and definition example.
  @@ -187,6 +188,16 @@ const Genders = {
  @prop({ enum: Object.values(Genders) })
  gender?: Gender;
  +
  +
  +// TS 2.4+ string enum example
  +enum Gender {
  + MALE = 'male',
  + FEMALE = 'female',
  +}
  +
  +@prop({ enum: Gender })
  +gender?: Gender;
  ```
  - `default`: The provided value will be the default for that Mongoose property.
  