Create cache.md

Author: propelledanalytics

docs/cache.md

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+# cache Function
+
+Caches Spark Dataset into memory.
+
+# Arguments
+- `ds::Dataset`: Dataset to cache.
+
+# Example
+```
+stmt = sql(session, "SELECT _c0 AS columnName1, _c1 AS columnName2 FROM CSV.`/pathToFile/fileName.csv`")
+cache(stmt)
+```