Notes concerning design added to README.

README.md

@@ -6,14 +6,27 @@ projects are implemented in a single language. Thus a tool was developed which
 provided a framework to aid in delivering support for multiple languages and
 multiple style choices.
 
-The project is still in its infancy so only a few rules have been implemented
-and only for fortran.
-
 The framework supports multiple styles, each consisting of multiple rules.
 Each of these may be one of two types, those which treat the source as a text
 file and those which treat it as a parse tree.
 
+The project is still in its infancy so only a few rules have been implemented
+and only for fortran.
+
+## Design
+
 In the past we have used text based tools and there's a surprising amount you
 can do with regular expressions. However once context becomes important they
 can become a liability. In those cases it is much better to have the parse
 tree.
+
+At the moment the tool is only able to flag up where a style rule has been
+broken. For some things this is the best we can hope for but for others
+we might aspire to correct the problem.
+
+For instance the current check for missing `implicit none` statements might,
+in the future, insert them where they are missing.
+
+Some UML class diagrams have been prepared which may be found in the
+`documentation` directory. These make use of the
+[PlantUML](https://plantuml.com) tool to render.