Create a simple migration guide

[adespawn]

Currently, no document indicates which parts of the driver work the same way as in the DataStax driver, which options are supported, and so on...

Add some initial documentation on how to migrate from the old driver, which will include a summary of all changes.

List of changes that are relevant:

• Consider updating default encoder options #346
• Document changes to LoadBalancing policies #364
• Update encoder options #369
• Built in retry policies #387 (Updated the default policy)
• TODO: add more as they appear

[adespawn]

Load balancing: mention that we recomend using default

[adespawn]

Retry: mention about new one

[adespawn]

Question:

Do we want to list all of the removed API in the migration guide, or would it be sufficient to provide some kind of API doc (like the once we use internally) listing all supported / removed APIs?

[wprzytula]

Question:

Do we want to list all of the removed API in the migration guide, or would it be sufficient to provide some kind of API doc (like the once we use internally) listing all supported / removed APIs?

This is what we settled on in C# RS Driver. I believe it's worth mentioning all removed APIs in the migration guide.