Improve UX of gen-random; add note to readme

README.md

@@ -98,6 +98,7 @@ The reason to pick numbers at random is to minimize risk of collisions
 in the wild. Do not pick random numbers by banging on keyboard, or by using
 `0xdeadf00d`, `0x42424242` etc. A good way is to use the following
 shell command: `printf "0x%04x%04x\n" $RANDOM $RANDOM`
+Another good way is the link at the bottom of https://microsoft.github.io/uf2/patcher/
 This procedure was unfortunately not used for the SAMD51 and NRF52840 below.
 
 #### Family list

patcher/web.js

@@ -5,7 +5,8 @@ function savePatch(ev) {
     localStorage["UF2_PATCH"] = text.value
 }
 
-function genRnd() {
+function genRnd(ev) {
+    ev.preventDefault()
     let vals = new Uint32Array(1)
     window.crypto.getRandomValues(vals)
     document.getElementById("rnd-res").textContent = "Random number: 0x" +