Document include guard convention

Author: practicalswift

doc/developer-notes.md

@@ -514,6 +514,16 @@ namespace {
     source file into account. This allows quoted includes to stand out more when
     the location of the source file actually is relevant.
 
+- Use include guards to avoid the problem of double inclusion. The header file
+  `foo/bar.h` should use the include guard identifier `BITCOIN_FOO_BAR_H`, e.g.
+
+```c++
+#ifndef BITCOIN_FOO_BAR_H
+#define BITCOIN_FOO_BAR_H
+...
+#endif // BITCOIN_FOO_BAR_H
+```
+
 GUI
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