Updated main README with CMake support info and changed docs extension from .txt to .md

docs/README.md

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+# JUCE Documentation
+
+This directory contains files documenting the JUCE Module Format, and the JUCE
+CMake API.
+
+The JUCE modules themselves can be found in the `modules` subdirectory of the
+JUCE repository.
+
+CMake example projects are located in the `examples/CMake` directory.
+
+The JUCE API itself is documented inline, but HTML docs can be generated from
+the source code using the `doxygen` tool. These HTML docs can be [found
+online](https://juce.com/learn/documentation), or you can generate a local copy
+which can be used without an internet connection. For instructions on generating
+offline docs, see below.
+
+# Generating Offline HTML Documentation
+
+## Dependencies
+
+- doxygen
+- python
+- make
+- graphviz (to generate inheritance diagrams)
+
+Make sure that all the dependencies can be found on your PATH.
+
+## Building
+
+- cd into the `doxygen` directory on the command line
+- run `make`
+
+Doxygen will create a new subdirectory "doc". Open doc/index.html in your browser
+to access the generated HTML documentation.