add doc example for implementing BinRead

[ScanMountGoat]

Adds a slightly non trivial example for how to implement BinRead. I found that implementing BinRead for a generic type with the recent args changes was straightforward but not very obvious without examples.

[csnover]

Hi, sorry about the delay. Wondering if it would be possible to do this example not using the FilePtrArgs type? It might be a silly thing to request since these are technically part of the public API but at the moment my gut says it isn’t a good idea to put examples in the docs that use these associated types that are only intended to be used with their parent class.

[ScanMountGoat]

I can change it to use a custom args type like CustomPtr32Args with the same fields.

[ScanMountGoat]

Do the latest changes look ok? @csnover

[ScanMountGoat]

Any updates on this? @csnover

[ScanMountGoat]

Does this look ok? @csnover