Add documentation for extension

src/DMS/Bundle/TwigExtensionBundle/Resources/doc/pad_string.md

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+# Pad String Extension
+
+The pad string extension uses PHP's native str_pad function
+to pad a string with a given character on the left, right or
+both sides until the returned string reaches the specified
+maximum length.
+
+## Usage
+
+This extension accepts three parameters; a pad character, an
+integer specifying the maximum length of the returned string
+and a string (optional) specifying which side to apply the
+padding:
+
+`{{ myString|padString('o', 4, 'STR_PAD_LEFT }}`
+`{{ myString|padString('o', 4, 'STR_PAD_BOTH }}`
+`{{ myString|padString('o', 4, 'STR_PAD_RIGHT }}`
+`{{ myString|padString('o', 4 }}`
+
+If myString contained the value 'ps', the output of the above
+would be:
+
+* oops
+* opso
+* psoo
+* psoo
+
+Where no third argument is provided, the filter defaults to
+STR_PAD_RIGHT.