docs/manual: document D language support

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/adding-packages-directory.txt

@@ -322,6 +322,10 @@ use in the comment.
 ** Dependency symbol: +BR2_INSTALL_LIBSTDCPP+
 ** Comment string: `C++`
 
+* D support
+** Dependency symbol: +BR2_TOOLCHAIN_HAS_DLANG+
+** Comment string: `Dlang`
+
 * Fortran support
 ** Dependency symbol: +BR2_TOOLCHAIN_HAS_FORTRAN+
 ** Comment string: `fortran`