It wasn't immediately clear to me where to put the cert and private key when I went through these instructions recently, so I thought I'd propose a change to make the destination explicit.

Closes #1103

* Cleaned up .gitignore a little bit

* Added a .gitignore to modules, ignoring all non-tracked modules.

* Fix internal markdown links in the install docs

* IDP configurable AuthnContextClassRef/AuthnContextComparison

* Add a few namespaces

* Update Psalm

* Ignore DocblockTypeContradiction and RedundantConditionGivenDocblockType

* Fix Psalm-errors

* Raise minimum PHP-version to 5.6

* Update lock-file accordingly

* Fix phpunit

* Suppress Psalm-issue

Some metadata publishers offer SHA256 fingerprints to validate the
fingerprint of the certificate used to sign metadata. Allow users to
configure the fingerprinting algorithm to facilitate validation.

For backward compatibility, the default algorithm remains SHA1.