docs/README.md: add instructions for regenerating doxygen docs

Author: bxparks

docs/README.md

@@ -1,7 +1,24 @@
 # Documentation
 
-* The `html` directory contains the programmatically generated
-  [Doxygen docs](https://bxparks.github.io/AceTime/html/) which are viewable on
-  GitHub Pages.
-* The other files are various subsections of the User Guide which are linked
-  from the main [README.md](../README.md).
+The main [README.md](../README.md) and [USER_GUIDE.md](../USER_GUIDE.md)
+contain the majority of the documentation.
+
+This directory contains scripts to generate [doxygen](https://www.doxygen.nl/)
+documentation from the embedded docstrings in the code.
+
+First, install doxygen and GNU Make if you don't already have them, with
+something like the following on an Ubuntu Linux machine:
+
+```
+$ sudo apt install doxygen make
+```
+
+Second, run the `make` command to generate the HTML files under the `html`
+directory:
+
+```
+$ cd docs
+$ make
+```
+
+Third, open the `./docs/html/index.html` file in your web browser.

docs/doxygen.cfg

@@ -38,7 +38,7 @@ PROJECT_NAME           = AceTime
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 3.0.0
+PROJECT_NUMBER         = 4.0.0
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a