Create DESIGN.md

Initial start with terminology and types as a place to begin evolving the design of RxJava v2.

I have bootstrapped this doc with things that should not be controversial, just statements of fact of what exists. If any are controversial, I apologize, please open a PR with the recommended change.

Further proposals to this document will come via PRs for discussion and approval.

Author: benjchristensen

DESIGN.md

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+## RxJava v2 Design
+
+Terminology, principles, contracts, and other aspects of the design of RxJava v2.
+
+### Terminology & Definitions
+
+##### Hot
+
+When used to refer to a data source (such as an `Observable`), it means it does not have side-effects when subscribed to.
+
+For example, an `Observable` of mouse events. Subscribing to that `Observable` does not cause the mouse events, but starts receiving them.
+
+(Note: Yes, there are *some* side-effects of adding a listener, but they are inconsequential as far as the 'hot' usage is concerned).
+
+##### Cold
+
+When used to refer to a data source (such as an `Observable`), it means it has side-effects when subscribed to.
+
+For example, an `Observable` of data from a remote API (such as an RPC call). Each time that `Observable` is subscribed to causes a new network call to occur.
+
+##### Reactive
+
+Producer is in charge. Consumer has to do whatever it needs to keep up.
+
+##### Interactive
+
+Consumer is in charge. Producer has to do whatever it needs to keep up.
+
+##### Push
+
+Producer emits when it wishes to. Related to "reactive". Callbacks are an instance of push.
+
+##### Pull
+
+Consumer requests data when it wishes to. Related to "interactive". An `Iterable` is an instance of pull.
+
+##### Async Pull
+
+Consumer requests data when it wishes, and the data is then pushed when the producer wishes to. The Reactive Streams `Publisher` is an instance of "async pull", as is the 'AsyncEnumerable' in .Net.
+
+### RxJava & Related Types
+
+##### Observable
+
+... under discussion ... (related to Observable/Flowable debate)
+
+##### Observer
+
+Consumer of events without flow control.
+
+##### Publisher
+
+[Reactive Streams producer](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.0/README.md#1-publisher-code) of data
+
+##### Subscriber
+
+[Reactive Streams consumer](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.0/README.md#2-subscriber-code) of data.
+
+##### Subscription
+
+[Reactive Streams state](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.0/README.md#3-subscription-code) of subscription supporting flow control and cancellation.
+
+##### Processor
+
+[Reactive Streams operator](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.0/README.md#4processor-code) for defining behavior between `Publisher` and `Subscriber`. It must obey the contracts of `Publisher` and `Subscriber`, meaning it is sequential, serialized, and must obey `request(n)` flow control.
+
+##### Subject
+
+A "hot" data source that allows a producer to emit events and consumers to receive events in a multicast manner.
+
+It is "hot" because consumers subscribing to it does not cause side-effects, or affect the data flow in any way. It is push and reactive because the producer is fully in charge.
+
+##### Disposable
+
+A type representing work that can be cancelled or disposed.
+
+