Update documentation

- Rewrite README.md
- Modify build-pdf.sh script to concatenate template and README.md
- Add userguide recipe to Makefile

docs/img/flowchart.md

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+flowchart TB
+    access[Access site]
+    activation[Activation]
+    valid[Valid key?]
+    remaining[Activations remaining?]
+    checkout[Stripe Checkout]
+    questionnaire[Questionnaire]
+    activate[Activate key]
+    results[Show table of results]
+    error[Error]
+    physical[Ordered physical copy?]
+    receive[Receive physical copy in mail]
+    done[Done]
+
+    access-->activation
+    activation-->checkout
+    checkout-->activation
+    activation-->valid
+    valid--"No"-->error
+    valid--"Yes"-->remaining
+    remaining--"Yes"-->questionnaire
+    remaining--"No"-->results
+    error-->activation
+    questionnaire-->activate
+    activate-->results
+    results-->physical
+    physical--"No"-->done
+    physical--"Yes"-->receive
+    receive-->done