Document the use of the summary function in readme/intro vignette

[tomjemmett]

The solution for #164 already exists in the form of the summary() method. However, this method is not mentioned in the Readme or intro vignette.

We could do with a paragraph in both of these showing how you can use summary to get some basic statistics about your spc data frame, and how this could be used (summary() prints the spc options, and then returns the summarised results. The main usage of summary() is in an interactive case, so this summarised table is typically also printed, but you could assign this table to a variable and use it later).

[marcosfabietti]

#198

created pull request for it, please review