README: use a better example

Some users were repeating the regexp compile, so make that more obvious.
Thanks to Kevin Burke for the idea.

While at it, update the go.mod language version.

Fixes #31.

README.md

@@ -8,10 +8,13 @@ Extract urls from text using regular expressions. Requires Go 1.12 or later.
 import "mvdan.cc/xurls/v2"
 
 func main() {
-	xurls.Relaxed().FindString("Do gophers live in golang.org?")
-	// "golang.org"
-	xurls.Strict().FindAllString("foo.com is http://foo.com/.", -1)
-	// []string{"http://foo.com/"}
+	rxRelaxed := xurls.Relaxed()
+	rxRelaxed.FindString("Do gophers live in golang.org?")  // "golang.org"
+	rxRelaxed.FindString("This string does not have a URL") // ""
+
+	rxStrict := xurls.Strict()
+	rxStrict.FindAllString("must have scheme: http://foo.com/.", -1) // []string{"http://foo.com/"}
+	rxStrict.FindAllString("no scheme, no match: foo.com", -1)       // []string{}
 }
 ```
 
@@ -21,7 +24,7 @@ Note that the funcs compile regexes, so avoid calling them repeatedly.
 
 To install the tool globally:
 
-	go get -u mvdan.cc/xurls/cmd/xurls
+	go get mvdan.cc/xurls/cmd/xurls
 
 ```shell
 $ echo "Do gophers live in http://golang.org?" | xurls

go.mod

@@ -1,3 +1,3 @@
 module mvdan.cc/xurls/v2
 
-go 1.10
+go 1.13