Add documentation around overriding `initWithDictionary:error:`

Getting started with Mantle, I ran into the same confusion as #336 and wrote up some notes.
This example is a little flawed, take the following use:

```objective-c
GHIssue *issue = [[GHIssue alloc] initWithDictionary:@{@"name": @"Initial value"}];
assert(issue.titleLength == 13);

issue.title = @"New updated value";
assert(issue.titleLength == 17); // Fails, it's actually 13
```