Add test and update README.

Author: Sean1708

test/runtests.jl

@@ -214,6 +214,13 @@ r = query(db, "SELECT doublesum(UnitPrice) FROM Track")
 s = query(db, "SELECT UnitPrice FROM Track")
 @test_approx_eq r[1][1] 2*sum(s[1])
 
+mycount(p, c) = p + 1
+mycount(p) = p
+register(db, 0, mycount, mycount)
+r = query(db, "SELECT mycount(TrackId) FROM PlaylistTrack")
+s = query(db, "SELECT count(TrackId) FROM PlaylistTrack")
+@test r[1] == s[1]
+
 db2 = SQLiteDB()
 query(db2, "CREATE TABLE tab1 (r REAL, s INT)")