Reflected those changes in unit design doc

design/units.md

@@ -29,6 +29,27 @@ there is an additional directory in the search path, such as
 `/usr/local/include/fspl`, then the unit will be searched for in each one (in
 order) until it is found.
 
+There are standard paths that the compiler will search for units in. These are,
+in order of preference:
+- `$HOME/.local/include/fsplc`
+- `$HOME/.local/src/fsplc`
+- `/usr/local/include/fsplc`
+- `/usr/local/src/fsplc`
+- `/usr/include/fsplc`
+- `/usr/src/fsplc`
+
+Files in `include` directories should *not* include program code, and should
+only define types and external functions and methods, similar to header files in
+C. They may have a corresponding shared object file that programs can
+dynamically link against.
+
+Files in `src` directories *may* contain program code, and may be compiled into
+an object file if the user wishes to link them statically. Because of FSPL's
+ability to "skim" units (discussed later in this document), files in `src` may
+be used in the same way that files in `include` are. Thus, `src` files are
+effectively more "complete" versions of `include` files with extended
+capability, and that is why they are searched first.
+
 ## Uniqueness
 
 Each unit is uniqued by a UUID. Most users will never directly use UUIDs, but