Add PGP public key and fix engine versions in package.json (#583)

* nodejs 17.9.0 also works

* Add github sponser link

* Add Public PGP key of signed tarball

* Update README.md

* Add download badge also.

* Add docker pulls

package.json

@@ -4,8 +4,8 @@
   "description": "Yet another matrix client",
   "main": "index.js",
   "engines": {
-    "npm": ">=6.14.8 <8.5.5",
-    "node": ">=14.15.0 <17.9.0"
+    "npm": ">=6.14.8 <=8.5.5",
+    "node": ">=14.15.0 <=17.9.0"
   },
   "scripts": {
     "start": "webpack serve --config ./webpack.dev.js --open",