[Dev] HISTORY.txt

[Ducky Sherwood]

At Heikki's request, I've started maintaining HISTORY.txt.  Neither of 
us are really sure who HISTORY.txt's customers are -- end users or 
developers? -- so it's unclear how much detail to put in.

For example, in this release, we didn't put in that Chandler now always 
runs RefCounted so pinning isn't required.  That's clearly an 
implementation detail, something that end-users wouldn't care about.  
However, if we had a third-party parcel developer who was pinning code,  
s/he might want to know about that.

So: what do we collectively want from HISTORY.txt?