[coverage-html] Add comment to expect installation.
[coverage-html] Add comment to expect installation.

melya/phpseclib,paragonie-scott/phpseclib,phpseclib/phpseclib-php5,phpseclib/phpseclib-php5,splitice/phpseclib,creatuity/phpseclib,creatuity/phpseclib,thaigialai1987/phpseclib,metaclassing/phpseclib,terrafrost/phpseclib,thaigialai1987/phpseclib,phpseclib/phpseclib,ericariyanto/phpseclib,kenichiii/phpseclib,melya/phpseclib,ericariyanto/phpseclib,paragonie-scott/phpseclib,danog/phpseclib,kenichiii/phpseclib,splitice/phpseclib,metaclassing/phpseclib,danog/phpseclib