Update prerequisites (#15)

Author: mfisher87

01-prerequisites.md

@@ -1,13 +1,41 @@
 # 🚦 Prerequisites
 
-## A laptop!
+This workshop requires a few prerequisites:
 
-**If you're using Windows, we highly recommend coming prepared with Windows Subsystem for
-Linux pre-configured.**
-This workshop uses `bash` for commands, but everything will also work with `zsh` (the
-new default for MacOS)!
 
+## A computer with a web browser installed
 
-## More
+All activities in this workshop will be performed in a cloud computing environment
+called [CryoCloud](https://cryointhecloud.com/).
 
-<TODO>
+**The only thing you need to have installed to participate is a web browser**!
+This means you can participate even on a Chromebook or a locked-down corporate computer.
+
+
+## A GitHub account
+
+Please [sign up for a GitHub account](https://github.com/signup) if you haven't already.
+
+
+## Complete the CryoCloud onboarding process
+
+Follow the
+[CryoCloud "getting started" documentation](https://book.cryointhecloud.com/getting-started)
+to gain access to CryoCloud.
+
+
+### ⚠️ Critical: Accept the invitation to join the CryoCloud team
+
+Once we have your github ID, we will send you an invitation to join a GitHub team that
+will give you access to log in to CryoCloud.
+**You must accept that invitation within 7 days!**
+
+* [Click here to accept your invitation](https://github.com/orgs/CryoInTheCloud/invitation).
+  You should see a simple page with a big green button to accept.
+  If you don't, it's possible you already accepted your invitation, possibly by e-mail.
+* [Log in to CryoCloud](https://hub.cryointhecloud.com/).
+  If you see a "403 Forbidden" error, you don't have the correct access.
+  Let a workshop organizer know, and give them your GitHub username, and we'll fix it
+  right up!
+  * You will see a dialog to "Authorize nasa-cryo-prod" on your first login.
+    **Click the green "Authorize 2i2c-org" button**.