Add mascot demo. Update assets for readme

.github/assets/bch-qr-placeholder.svg

@@ -1,11 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" width="640" height="640" viewBox="0 0 640 640" role="img" aria-labelledby="title desc">
-  <title id="title">BCH QR placeholder</title>
-  <desc id="desc">Placeholder card reminding the maintainer to replace this with a real BCH QR code.</desc>
-  <rect width="640" height="640" rx="32" fill="#111111"/>
-  <rect x="64" y="64" width="512" height="512" rx="24" fill="#ffffff"/>
-  <path d="M156 156h128v128H156zM356 156h128v128H356zM156 356h128v128H156z" fill="#111111"/>
-  <path d="M192 192h56v56h-56zM392 192h56v56h-56zM192 392h56v56h-56z" fill="#ffffff"/>
-  <path d="M332 340h32v32h-32zM396 340h32v32h-32zM460 340h32v32h-32zM332 404h32v32h-32zM460 404h32v32h-32zM364 436h32v32h-32zM428 436h32v32h-32z" fill="#111111"/>
-  <text x="320" y="560" text-anchor="middle" fill="#111111" font-family="Menlo, Monaco, monospace" font-size="24">REPLACE WITH REAL BCH QR</text>
-  <text x="320" y="602" text-anchor="middle" fill="#ffffff" font-family="Menlo, Monaco, monospace" font-size="24">ADD YOUR bitcoincash: ADDRESS</text>
-</svg>