manual: document minimal kernel headers dependency

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-directory.txt

@@ -218,6 +218,12 @@ use in the comment.
 ** Dependency symbol: +BR2_USE_MMU+
 ** Comment string: no comment to be added
 
+* Kernel headers
+** Dependency symbol: +BR2_TOOLCHAIN_HEADERS_AT_LEAST_X_Y+, (replace
+   +X_Y+ with the proper version, see +toolchain/toolchain-common.in+)
+** Comment string: +headers >= X.Y+ (replace +X.Y+ with the
+   proper version)
+
 * C library
 ** Dependency symbol: +BR2_TOOLCHAIN_USES_GLIBC+,
 +BR2_TOOLCHAIN_USES_UCLIBC+