doc/start_pup: add install instructions

doc/api/start_pup.rst

@@ -1,16 +1,62 @@
 Powered Up Quick Start
 ########################
 
-Coming soon!
+This page shows how you can install Pybricks on your hub and run your first
+program. When you're ready, browse through the Pybricks Modules in the left
+hand menu to modify the example program.
+
+.. warning::
+
+    - **This is still in beta! Not everything may work as expected yet.**
+
+    - **As with any beta software, proceed with caution.**
+
+    - At the moment, only the **Control+ Hub** is ready for beta use.
+
+    - We welcome your questions, feedback and bug reports on our `support page`_.
+
+Preparing your computer
+--------------------------------
+
+Make sure that Bluetooth is enabled on your device and go to
+`code.pybricks.com`_.
+
+Right now, we officially support only Google Chrome, *version 85 or newer*.
+It is currently available as a beta/developer version. You can install
+it on Windows, Mac, and Linux without affecting your existing browser or data.
+Download it `from here`_.
+
+.. toggle-header::
+    :header: **Extra steps for Linux users**
+
+    - In Google Chrome or Chromium, go to ``chrome://flags/``.
+    - Search for *Experimental Web Platform features*.
+    - Enable it.
+
 
 
 Installing the Pybricks firmware
 --------------------------------
 
+Pybricks uses an improved firmware that can run autonomous programs.
+Installing it is easy:
+
 .. raw:: html
 
     <video controls src="http://pybricks.com/wp-content/uploads/2020/06/install.mp4" width="100%"></video>
 
+If you can't want watch the video, just follow these steps:
+
+- Make sure your batteries are fresh.
+- Make sure the hub is off.
+- Press and hold the green button.
+- Wait for the light to blink, but keep holding the button.
+- On the Pybricks Code page, click the firmware update button.
+- Select the ``LEGO Bootloader`` and click *Pair*.
+- The light should change to a red/green/blue sequence.
+- You can now release the button.
+- Wait for the update to finish, indicated by a steady blue light.
+- If you use the recommended browser, this will take about 90 seconds.
 
 Running programs
 --------------------------------
@@ -76,3 +122,6 @@ try again with fresh batteries.
 
 
 .. _latest builds: https://github.com/pybricks/pybricks-micropython/actions?query=workflow%3ABuild+
+.. _support page: https://github.com/pybricks/support/issues/
+.. _code.pybricks.com: http://code.pybricks.com/
+.. _from here: https://www.google.com/chrome/dev/