update README.dev

README.dev

@@ -14,9 +14,19 @@ To add new filetype foo:
 -  #include foo.h in xylib.cpp
 -  add an entry to formats[] array in xylib.cpp (but not after TextDataSet)
 -  add foo.cpp and foo.h files to xylib/Makefile.am
+   and/or add foo.cpp to CMakeLists.txt
 
 Do not worry too much about metadata, especially if the file is human-readable.
 Currently programs programs that use xylib mostly ignore metadata.
 
 You may contact us before you start, to avoid duplication of work.
 
+
+METADATA CONVENTIONS
+====================
+
+Naming conventions are introduced and documented here when programs using
+this library start relying on it.
+
+"source energy" (block) - energy of photons used in XPS
+