Add to docs

Author: cpfiffer

docs/src/api.md

@@ -34,11 +34,11 @@ AbstractMCMC.sample(
 )
 ```
 
-Two algorithms are provided for parallel sampling with multiple threads and multiple processes,
-respectively:
+Two algorithms are provided for parallel sampling with multiple threads and multiple processes, and one allows for the user to sample multiple chains in serial (no parallelization):
 ```@docs
 AbstractMCMC.MCMCThreads
 AbstractMCMC.MCMCDistributed
+AbstractMCMC.MCMCSerial
 ```
 
 ## Common keyword arguments