Add skeleton of ir-bm25.

GNUmakefile

@@ -7,9 +7,12 @@ DIST?=	dist/
 GO?=	go
 
 #
-all: dist/ir-tfidf
+all: dist/ir-bm25 dist/ir-tfidf
 	@true
 
+dist/ir-bm25: cmd/ir-bm25/*
+	"${GO}" build -o "${DIST}" ./cmd/ir-bm25
+
 dist/ir-tfidf: cmd/ir-tfidf/*
 	"${GO}" build -o "${DIST}" ./cmd/ir-tfidf
 

cmd/ir-bm25/main.go

@@ -0,0 +1,4 @@
+package main
+
+func main() {
+}