Add the first of the hyperlink docs

docs/HyperlinkedInformation/Diffs.md

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+## Only Before and Only After 
+
+This document briefly explains what you are seeing in those mysterious sections
+labelled "Only Before" and "Only After".  Enlightenment is nigh.
+
+`re-frame-trace` displays important values like, for example, what's
+currently in `app-db` or in a given subscription. While current values are good,
+you are often more interested to know how a value has been changed by the epoch. 
+The value might start of as, say, `X` (before the event happened) but 
+ended up as `Y`.  How are `X` and `Y` different, you wonder? 
+What got added or removed? What was modified? 
+
+So, how then to display changes in a way that's easy to grok?  I'm glad you asked. 
+`re-frame-trace` chooses to use the output of `(clojure.data/diff X Y)`.  Remember
+`X` is the before (original) value, and `Y` is the final value. 
+
+By [looking at the docs](https://clojuredocs.org/clojure.data/diff) on `diff`, you'll see
+that it returns a triple of values. `re-frame-trace`  
+displays the first two elements of this triple as "only before" and "only after" respectively.
+The 3rd element is not very interesting because it tells us what hasn't changed, so it isn't shown.  
+
+To correctly interpret "Only Before" and "Only after", you'll need to spend a bit 
+of time properly familiarising yourself with how `clojure.data/diff` works, but
+it will be a worthwhile investment. 

docs/HyperlinkedInformation/README.md

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+This directory holds help documents accessed directly from the UI. 
+
+The `re-frame-trace` UI contains 
+various (helpful) hyperlinks which point to documents kept within this directory.