Document upgrade procedure in FREEBSD-upgrade

It was pointed out to me that this is the convention for documenting upgrade
instructions, rather than just leaving the instructions in the commit message.
It's possible these commands won't be used again before we transition to git,
but then at least they'll give a path forward for whoever touches this next.

Suggested by:	lwhsu

sys/contrib/edk2/FREEBSD-upgrade

@@ -0,0 +1,23 @@
+$FreeBSD$
+
+We try to maintain the minimal set of headers required to build, as the full
+set of files from MdePkg is quite large (10MB at the time of writing). To do
+this when performing an upgrade, execute the following:
+
+# Generate list of the headers needed to build
+cp -r ../vendor/edk2/dist/MdePkg/Include sys/contrib/edk2
+cd lib/libefivar
+make
+pushd `make -V .OBJDIR`
+cat .depend*.o | grep sys/contrib | cut -d' ' -f 3 |
+    sort -u | sed -e 's=/full/path/sys/contrib/edk2/==' > /tmp/xxx
+popd
+
+# Merge the needed files
+cd ../../sys/contrib/edk2
+svn revert -R .
+for i in `cat /tmp/xxx`; do
+    svn merge -c VendorRevision svn+ssh://repo.freebsd.org/base/vendor/edk2/dist/MdePkg/$i $i
+done
+svn merge -c VendorRevision \
+    svn+ssh://repo.freebsd.org/base/vendor/edk2/dist/MdePkg/MdePkg.dec MdePkg.dec