Make the memory interface merging docs more explicit.

[ahmedcharles]

[dmarcuse]

It doesn't need to be in a .d.ts, just in an ambient context. This means it can be:

1. In a non-module source file (.ts without any imports or exports) in the form declare let thing: ...
2. In a module source file (.ts with imports or exports) in the form declare global { let thing: ... }
3. In a declaration file (.d.ts) in the form declare let thing: ...

[ahmedcharles]

Well, I'm happy as long as the docs are improved so that new users don't have to go through this same thing again.

[Arcath]

Maybe the readme should link to https://github.com/screepers/screeps-typescript-starter/blob/master/src/types.d.ts and we put an example of CreepMemory in there?

[resir014]

@ahmedcharles Suggested change:

Example:

[code snip]

**Note:** This code needs to be in a file with a `.d.ts` extension and
the name cannot match any of the other base names in the same directory,
i.e. `memory.d.ts` without an adjecent `memory.ts`. See
[the declarations on the TypeScript starter kit](https://github.com/screepers/screeps-typescript-starter/blob/master/src/types.d.ts)
for example.