update docs for libre kernel

docs/Libre-Kernel.md

@@ -42,6 +42,7 @@ In `/etc/portage/package.unmask`:
 ```
 sys-kernel/libre-kernel::projectmoon
 virtual/dist-kernel::projectmoon
+virtual/linux-sources::projectmoon
 sec-keys/openpgp-keys-linux-libre::projectmoon
 ```
 
@@ -51,6 +52,7 @@ In `/etc/portage/package.accept_keywords`:
 sys-kernel/librem-ec-acpi::projectmoon
 sys-kernel/libre-kernel::projectmoon ~amd64
 virtual/dist-kernel::projectmoon
+virtual/linux-sources::projectmoon
 ```
 
 In `/etc/portage/package.mask`:

virtual/linux-sources/linux-sources-3-r6.ebuild

@@ -0,0 +1,26 @@
+# Copyright 1999-2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+
+DESCRIPTION="Virtual for Linux kernel sources"
+SLOT="0"
+KEYWORDS="~alpha amd64 arm arm64 ~hppa ~ia64 ~loong ~m68k ~mips ppc ppc64 ~riscv ~s390 sparc x86"
+IUSE="firmware"
+
+RDEPEND="
+	firmware? ( sys-kernel/linux-firmware )
+	|| (
+		sys-kernel/gentoo-sources
+		sys-kernel/vanilla-sources
+		sys-kernel/git-sources
+		sys-kernel/mips-sources
+		sys-kernel/pf-sources
+		sys-kernel/rt-sources
+		sys-kernel/zen-sources
+		sys-kernel/raspberrypi-sources
+		sys-kernel/gentoo-kernel
+		sys-kernel/gentoo-kernel-bin
+		sys-kernel/vanilla-kernel
+		sys-kernel/libre-kernel
+	)"