Added mention to the minimum php version

I fell into this trap as there was no php version presumed 7.0 was or would be OK.  Luckily my dev server caught the error. So thought best to have it visually available for future users as its a good little package.

Author: simondavies

README.md

@@ -9,6 +9,10 @@ This package lets you generate entity relation diagrams by inspecting the relati
 It is highly customizable.
 Behind the scenes, it uses [GraphViz](https://www.graphviz.org) to generate the graph.
 
+## Prerequisites
+
+The minimum required PHP version is 7.1.0.
+
 ## Requirements
 
 This package requires the `graphviz` tool.