Document bzr as a build dependency in README.md

While fetching some other code during `make`, I got the following errors:

```
mkdir -p /home/me/repos/trousseau/.gopath/src/github.com/oleiade/
go: missing Bazaar command. See http://golang.org/s/gogetcmd
package code.google.com/p/go.crypto/openpgp
	imports code.google.com/p/go.crypto/openpgp/armor
	imports code.google.com/p/go.crypto/ssh
	imports github.com/codegangsta/cli
	imports github.com/oleiade/reflections
	imports launchpad.net/goamz/aws: exec: "bzr": executable file not found in $PATH
package code.google.com/p/go.crypto/openpgp
	imports code.google.com/p/go.crypto/openpgp/armor
	imports code.google.com/p/go.crypto/ssh
	imports github.com/codegangsta/cli
	imports github.com/oleiade/reflections
	imports launchpad.net/goamz/s3
	imports launchpad.net/goamz/s3
	imports launchpad.net/goamz/s3: cannot find package "launchpad.net/goamz/s3" in any of:
	/usr/lib/go/src/pkg/launchpad.net/goamz/s3 (from $GOROOT)
	/home/me/repos/trousseau/.gopath/src/launchpad.net/goamz/s3 (from $GOPATH)
Makefile:43: recipe for target '/home/me/repos/trousseau/.gopath/src/github.com/oleiade/trousseau' failed
make: *** [/home/me/repos/trousseau/.gopath/src/github.com/oleiade/trousseau] Error 1
```

I am not sure if this problem is archlinux-specific (my linux distro) or if it is a general issue. Either way, a comment in the README.md wouldn't hurt :smiley: