build: Improve build instructions

- Add package instructions for Ubuntu 15.10
- Clarify BerkeleyDB/wallet situation for unix
- Add basic build instructions for Windows (closes #1401)

doc/README.md

@@ -43,6 +43,8 @@ The following are developer notes on how to build Bitcoin on your native platfor
 
 - [OS X Build Notes](build-osx.md)
 - [Unix Build Notes](build-unix.md)
+- [Windows Build Notes](build-windows.md)
+- [OpenBSD Build Notes](build-openbsd.md)
 - [Gitian Building Guide](gitian-building.md)
 
 Development