CODING_CONVENTIONS.md: Add IWYU policy

This makes an implicit agreement that code should include the headers
is uses, no more and no less, explicit.

It also recommends using tooling (such as clangd) and adding IWUY
pragma comments to aid those tools in common cases they otherwise would
provide false positives.

Co-authored-by: chrysn <[email protected]>

CODING_CONVENTIONS.md

@@ -175,6 +175,24 @@
 #endif
 ```
 
+### Include What You Use (IWYU)
+
+`#include` directives that are not actually needed should be removed to reduce
+clutter and improve compilation speed. Similar: Try to add the corresponding
+`#include`s for all the functions, macros, types, etc. used and do not rely on
+`bar.h` to implicitly include `foo.h`, unless this is documented behavior.
+
+Tools such as [clang's Include Cleaner][clangd-include-cleaner] can help with
+that. These tools may show false positives in cases where headers are *expected*
+to be included indirectly: E.g. if `foo.h` is the public header that contains
+common helpers and implementations, but a per platform `foo_arch.h` is included
+from within `foo.h` for platform specific implementations. If in this scenario
+only functions provided by `foo_arch.h` are included, the `#include` of `foo.h`
+is considered as unused. To avoid this, one should add
+[`/* IWYU pragma: export */`](https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md) after `#include "foo_arch.h"` in `foo.h`.
+
+[clangd-include-cleaner]: https://clangd.llvm.org/design/include-cleaner
+
 ## Header Guards
 
 All files are required to have header guards of the form