Get an extra pair of eyes on the Softlayer installation docs

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)

docs/sources/installation/MAINTAINERS

@@ -1 +1,2 @@
 google.md: Johan Euphrosine <proppy@google.com> (@proppy)
+softlayer.md: Phil Jackson <underscorephil@gmail.com> (@underscorephil)