Update README.md with usage information

README.md

@@ -11,16 +11,72 @@ Similar to Autoconf, this module provides built-in support for a number of
 common `HAVE_*` configuration options. However, the values of these options
 are not discovered by dynamic probing, such as trying to compile a test
 program to check if the feature is present. Instead, they are set to static
-expected values based on the platform/compiler macros (see note at the
+expected values based on the platform/compiler macro checks (see note at the
 beginning of [Project Configuration][proj-config] for rationale).
 
 See [`libbuild2/autoconf/checks/`][checks] for the list of available build-in
-configuration options.
+checks.
 
-## Adding new configuration options.
+## Using in your projects
 
-To add a new configuration option `<NAME>` simply create the `<NAME>.h` header
+This module is part of the standard pre-installed `build2` modules and no
-file with the corresponding check and place it into
+extra integration steps are required other than the `using` directive in
+your `buildfile`. For example, for Autoconf `config.h.in`:
+
+```
+using autoconf
+
+h{config}: in{config}
+```
+
+Or for CMake `config.h.cmake`:
+
+```
+using autoconf
+
+h{config}: in{config.h.cmake}
+```
+
+The default falvor is `autoconf` but if the input file has the `.cmake` or
+`.meson` extension, then the `cmake` or `meson` flavors are selected
+automatically.  If, however, the standard `config.h.in` file is re-used for
+CMake/Meson, then the flavor must be specified explicitly with the
+`autoconf.flavor` variable, for example:
+
+```
+using autoconf
+
+h{config}: in{config}
+{
+  autoconf.flavor = meson
+}
+```
+
+Besides the built-in configuration options, custom substitutions can be
+specified as `buildfile` variables in the same way as with the
+[`in`][module-in] module. For example:
+
+```
+h{config}: in{config}
+{
+  PACKAGE_NAME = $project
+  PACKAGE_VERSION = $version
+}
+```
+
+This mechanism can also be used to override the build-in checks, for example:
+
+```
+h{config}: in{config}
+{
+  HAVE_STRLCPY = true
+}
+```
+
+## Adding new checks
+
+To add a check for a new configuration option `<NAME>` simply create the
+`<NAME>.h` header file with the corresponding check and place it into
 [`libbuild2/autoconf/checks/`][checks] (use existing checks for inspiration).
 
 The first line in this header file must be in the form: