Merge pull request #219 from tskit-dev/doc_write_ups_214

Add edges page description for docs

docs/edges.md

@@ -2,6 +2,15 @@
 
 # Edges page
 
-TODO! Explain page
+![Edges Page](tsbrowse:example.tsbrowse:edges)
 
-![Edges Page](tsbrowse:example.tsbrowse:edges)
+An edge represents the relationship between a pair of nodes (parent, child) in the ARGs over time (assigning the node occurring in more recent time as the child). 
+
+The interactive plot on the top row allows us to visualise these relationships as horizontal lines denoting the length of the edge on the sequence (X-axis) and time of one of the nodes (Y-axis). On mouse-over of an individual edge, the tool-tip displays the parent and child node IDs, length of sequence the edge spans, and the amount of time it spans (branch length).
+
+The bottom panel contains three static histograms which. left to right, summarise the span of edges over the sequence, the span of edges over times (branch lengths), and the product of the two spans.
+
+
+## Plot controls (sidebar):
+* The `log y-axis` checkbox plots node time on log scale.
+* The checkbox group allows switching between the nodes represented on the Y-axis (parent or child).