Add 'path' and 'documentation' sections in README.md

README.md

@@ -44,6 +44,39 @@ with Pd's default extension for the platform. For anything more than the
 most basic usage, read the documentation sections in Makefile.pdlibbuilder.
 
 
+### paths ###
+
+
+Makefile.pdlibbuilder >= v0.4.0 supports pd path variables which can be
+defined not only as make command argument but also in the environment, to
+override platform-dependent defaults:
+
+PDDIR:
+Root directory of 'portable' pd package. When defined, PDINCLUDEDIR and 
+PDBINDIR will be evaluated as $(PDDIR)/src and $(PDDIR)/bin.
+
+PDINCLUDEDIR:
+Directory where Pd API m_pd.h should be found, and other Pd header files.
+Overrides the default search path.
+
+PDBINDIR:
+Directory where pd.dll should be found for linking (Windows only). Overrides
+the default search path.
+
+PDLIBDIR:
+Root directory for installation of Pd library directories. Overrides the
+default install location.
+
+
+### documentation ###
+
+
+This README.md provides only basic information. A large comment section inside
+Makefile.pdlibbuilder lists and explains the available user variables, default
+paths, and targets. The internal documentation reflects the exact functionality
+of the particular version. A tips&tricks page is in the works. 
+
+
 ### examples ###