Added release notes for 2.1.0-RC1.
Added release notes for 2.1.0-RC1.

fthomas/shapeless,wheaties/shapeless,khronnuz/shapeless,notxcain/shapeless,isaka/shapeless,tek/shapeless,clhodapp/shapeless,TomasMikula/shapeless,Atry/shapeless,malcolmgreaves/shapeless,roberth/shapeless,alexarchambault/shapeless,triggerNZ/shapeless,milessabin/shapeless,lukasz-golebiewski/shapeless,liff/shapeless,lambdista/shapeless,joescii/shapeless