[doc] Re-add README file

The README file was accidentally deleted in commit 2aad3fa ("[build]
Use weak definitions instead of weak declarations").

Signed-off-by: Michael Brown <mcb30@ipxe.org>

README

@@ -0,0 +1,8 @@
+iPXE README File
+
+Quick start guide:
+
+   cd src
+   make
+
+For any more detailed instructions, see http://ipxe.org