Add sed OSX documentation

sed.md

@@ -0,0 +1,17 @@
+---
+title: Sed
+layout: default
+---
+
+### OSX Caveat
+
+To do in place replacements `-i ''` is required (GNU/sed is different)
+
+    sed -i '' -e 's/foo/bar/' example.md
+
+
+### GNU/sed
+
+To do in place replacements use `-i` without arg
+
+    sed -i -e 's/foo/bar/' example.md