docs: Add a note about the source code filename naming convention

doc/developer-notes.md

@@ -567,6 +567,12 @@ Source code organization
 
   - *Rationale*: Shorter and simpler header files are easier to read, and reduce compile time
 
+- Use only the lowercase alphanumerics (`a-z0-9`), underscore (`_`) and hyphen (`-`) in source code filenames.
+
+  - *Rationale*: `grep`:ing and auto-completing filenames is easier when using a consistent
+    naming pattern. Potential problems when building on case-insensitive filesystems are
+    avoided when using only lowercase characters in source code filenames.
+
 - Every `.cpp` and `.h` file should `#include` every header file it directly uses classes, functions or other
   definitions from, even if those headers are already included indirectly through other headers.