add cacheTable guide

add the `cacheTable` specification
Some grammar fixes:

```
Spark SQL can cache tables using an in-memory columnar format by calling `cacheTable("tableName")`.
Then Spark SQL will scan only required columns and will automatically select best compression to
minimize memory usage an GC pressure. You can call `uncacheTable("tableName")` to remove the
table from memory.

Note that if you just call `cache` rather than `cacheTable`, tables will _not_ be cached in
in-memory columnar format. So we strongly recommend using `cacheTable` whenever you want to
cache tables.
```