Add a Module.md description for coming documentation on modules [#125]

doc/modules/Modules.md

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+# Modules
+
+The WebBlocks semantics are organized in a series of modules. Each module is a 
+logical subdivision that contains a number of definitions.
+
+Top-level modules includes:
+
+* `base` - Foundational elements that establish a grid, colors and provide 
+utilities for floating, visibility, image replacement and other such things. 
+This namespace should almost always be included in a WebBlocks build.
+* `entity` - Common user interface elements including styled versions of 
+HTML base elements elements such as lists, tables, blockquotes and buttons, as 
+well as more complex elements such as headers and navs. This namespace is 
+generally included in a WebBlocks build, and many of its components require the 
+`base` namespace.
+* `extend` - More advanced elements that enhance the user experience such as a 
+scrollable table. This namespace is not always included in full, but rather 
+individual modules are often selected. In many cases, elements within the 
+`extend` namespace require components from the `entity` namespace to work 
+correctly.
+
+During the build process, these modules may be enabled or disabled through the 
+`WebBlocks.config[:src][:modules]` directive. See the Configuration 
+documentation for more information.