DOC: Reword `readme`

[HaoZeke]

readme: "Unlike the Matrix package" : maybe rephrase? it is a bit off-putting for me to start the whole readme with a negative comparison, although
I understand that a core part of the motivation for this package is that Matrix does not support everything and not fast enough. Even changing the order of the two parts of the sentence would feel better for me but this is really personal

First noted here: ropensci/software-review#606 (comment).

[HaoZeke]

Also:

readme sentence: "Similarly, we support writing and reading from other R objects (e.g. standard R vectors and matrices), as seen in the basic use vignette."
Consider rephrasing a bit, e.g. refer to 'getting started' vignette instead of basic use, possibly as described in the vignette / as you can see in the vignette instead of as seen?