Added pushd.md

pages/osx/pushd.md

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+# pushd
+
+> Place a directory on a stack so it can be accessed later
+
+- Switch to directory and push it on the stack
+
+`pushd < {{directory}}`
+
+- Switch first and second directories on the stack
+
+`pushd`
+
+- Rotate stack by making the 5th element the top of the stack
+
+`pushd +4`