doc: Complement document about booting VxWorks

Current document about how to boot VxWorks is limited.
Add several chapters in README.vxworks to document this.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/doc/README.x86 b/doc/README.x86
index 6cf293b..a4f5321 100644
--- a/doc/README.x86
+++ b/doc/README.x86
@@ -25,6 +25,8 @@
 
 As for loading an OS, U-Boot supports directly booting a 32-bit or 64-bit
 Linux kernel as part of a FIT image. It also supports a compressed zImage.
+U-Boot supports loading an x86 VxWorks kernel. Please check README.vxworks
+for more details.
 
 Build Instructions for U-Boot as coreboot payload
 -------------------------------------------------