added docs

Author: torfjelde

docs/src/api.md

@@ -76,3 +76,16 @@ For chains of this type, AbstractMCMC defines the following two methods.
 AbstractMCMC.chainscat
 AbstractMCMC.chainsstack
 ```
+
+## Interacting with states of samplers
+
+To make it a bit easier to interact with some arbitrary sampler state, we encourage implementations of `AbstractSampler` to implement the following methods:
+```@docs
+AbstractMCMC.parameters(state, parameters)
+AbstractMCMC.setparameters!!(state, parameters)
+```
+and optionally
+```@docs
+AbstractMCMC.updatestate!!(state, transition, state_prev)
+```
+These methods can also be useful for implementing samplers which wraps some inner samplers, e.g. a mixture of samplers.