update documentation after API changes

doc/developers/interpolation.rst

@@ -0,0 +1,34 @@
+
+.. _interpolation:
+
+Interpolation
+=============
+
+In a custom extension to the YAML specification you can define and reuse
+variables like this (observe how we interpolate ``url_root``, ``major``,
+``minor``, ``patch``, and ``components`` in this example)::
+
+  url_root: https://github.com/coderefinery/autocmake/raw/master/
+  major: 1
+  minor: 48
+  patch: 0
+  components: ""
+  fetch:
+    - "%(url_root)modules/boost/boost_unpack.cmake"
+    - "%(url_root)modules/boost/boost_userconfig.cmake"
+    - "%(url_root)modules/boost/boost_configure.cmake"
+    - "%(url_root)modules/boost/boost_build.cmake"
+    - "%(url_root)modules/boost/boost_install.cmake"
+    - "%(url_root)modules/boost/boost_headers.cmake"
+    - "%(url_root)modules/boost/boost_cleanup.cmake"
+    - "http://sourceforge.net/projects/boost/files/boost/%(major).%(minor).%(patch)/boost_%(major)_%(minor)_%(patch).zip"
+  docopt:
+    - "--boost-headers=<BOOST_INCLUDEDIR> Include directories for Boost [default: '']."
+    - "--boost-libraries=<BOOST_LIBRARYDIR> Library directories for Boost [default: '']."
+    - "--build-boost=<FORCE_CUSTOM_BOOST> Deactivate Boost detection and build on-the-fly <ON/OFF> [default: OFF]."
+  define:
+    - "'-DBOOST_INCLUDEDIR=\"{0}\"'.format(arguments['--boost-headers'])"
+    - "'-DBOOST_LIBRARYDIR=\"{0}\"'.format(arguments['--boost-libraries'])"
+    - "'-DFORCE_CUSTOM_BOOST={0}'.format(arguments['--build-boost'])"
+    - "'-DBOOST_MINIMUM_REQUIRED=\"%(major).%(minor).%(patch)\"'"
+    - "'-DBOOST_COMPONENTS_REQUIRED=\"%(components)\"'"