README: update slack invite and badge

.github/slack.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="90" height="20">
+  <linearGradient id="smooth" x2="0" y2="100%">
+    <stop offset="0" stop-color="#bbb" stop-opacity=".1"/>
+    <stop offset="1" stop-opacity=".1"/>
+  </linearGradient>
+
+  <mask id="round">
+    <rect width="90" height="20" rx="3" fill="#fff"/>
+  </mask>
+
+  <g mask="url(#round)">
+    <rect width="42" height="20" fill="#555"/>
+    <rect x="42" width="48" height="20" fill="#E01563"/>
+    <rect width="90" height="20" fill="url(#smooth)"/>
+  </g>
+
+  <g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="11">
+    <text x="22" y="15" fill="#010101" fill-opacity=".3">slack</text>
+    <text x="22" y="14">slack</text>
+    <text x="65" y="15" fill="#010101" fill-opacity=".3">Join us!</text>
+    <text x="65" y="14">Join us!</text>
+  </g>
+</svg>
+

README.md

@@ -4,7 +4,7 @@
 
 [![Nuttx Targets](https://github.com/PX4/PX4-Autopilot/workflows/Nuttx%20Targets/badge.svg)](https://github.com/PX4/PX4-Autopilot/actions?query=workflow%3A%22Nuttx+Targets%22?branch=master) [![SITL Tests](https://github.com/PX4/PX4-Autopilot/workflows/SITL%20Tests/badge.svg?branch=master)](https://github.com/PX4/PX4-Autopilot/actions?query=workflow%3A%22SITL+Tests%22)
 
-[![Slack](https://px4-slack.herokuapp.com/badge.svg)](http://slack.px4.io)
+[![Slack](/.github/slack.svg)](https://join.slack.com/t/px4/shared_invite/zt-si4xo5qs-R4baYFmMjlrT4rQK5yUnaA)
 
 This repository holds the [PX4](http://px4.io) flight control solution for drones, with the main applications located in the [src/modules](https://github.com/PX4/PX4-Autopilot/tree/master/src/modules) directory. It also contains the PX4 Drone Middleware Platform, which provides drivers and middleware to run drones.