Add mercurial info to README

mercurial なしで go get したら怒られたので、その旨 README に追加しました

```
> go get github.com/yurufuwa/yurufuwa/cmd/yurufuwa
go: missing Mercurial command. See http://golang.org/s/gogetcmd
package github.com/yurufuwa/yurufuwa/cmd/yurufuwa
	imports github.com/codegangsta/cli
	imports github.com/google/go-github/github
	imports github.com/google/go-querystring/query
	imports github.com/satococoa/github-auth/client
	imports code.google.com/p/goauth2/oauth: exec: "hg": executable file not found in $PATH
go: missing Mercurial command. See http://golang.org/s/gogetcmd
package github.com/yurufuwa/yurufuwa/cmd/yurufuwa
	imports github.com/codegangsta/cli
	imports github.com/google/go-github/github
	imports github.com/google/go-querystring/query
	imports github.com/satococoa/github-auth/client
	imports github.com/howeyc/gopass
	imports code.google.com/p/go.crypto/ssh/terminal: exec: "hg": executable file not found in $PATH
```