Add Slither paper (WETSEB 19)

[montyly]

Hi, thank you for maintaining this repo!

Checklist for submitting a pull request to blockchain-papers:

• I have carefully read and comply with the Contributing Guidelines of this repo.
• I have searched the PRs of this repo and believe that this is not a duplicate.

[dimkarakostas]

@montyly please squash your commits in a single one to keep the repo history clean.

[montyly]

@dimkarakostas sure, sorry for the multiple commits. I force-pushed the modifications

[OrfeasLitos]

LGTM

Caught a small typo: p. 4, Fig. 2, line 5: min -> sub

Since this is a peer-reviewed, published work, it would be better to link to the original proceedings pdf. Unfortunately, judging from last year's proceedings, the paper will be behind a paywall.

• If this year there's no paywall, please remember to update the link to point there.
• If there's a paywall, the link you provided now is fine. But do not update the pdf, because it will not correspond to the published version. (If you want to update your paper, please place the newer version in another page.)

Thanks for your contribution!

[dionyziz]

Relevant: #98