Update readme on using multiSamlStrategy

[gugu]

passport-saml/multiSamlStrategy.js is something I'd like to remove:

1. it contains nothing, just import
2. it is outside of the source root and tsc/other tools don't include this file
3. old import depends on internal structure of the project
4. it uses module-level export

For now let's keep the file for compatibility, but let's update README so new people will not use the old way

[cjbarth]

I feel like we can remove that old file for a v3 release, which I believe is the next in line, so I say, remove it.

[gugu]

@cjbarth done

[markstos]

We just are getting to upgrading to 3.x ourselves and although I read through the CHANGELOG for breaking changes, I missed this. It could use a more clear note in the Changelog that it's a breaking change and a code change is required:

-var MultiSamlStrategy = require('passport-saml/multiSamlStrategy');
+const { MultiSamlStrategy } = require('passport-saml');

However, given the automated Changelog generation I'm not sure how to add that. At least if someone follows a link from the Changelog to here, they'll find the required syntax change more readily.

[cjbarth]

The PR that caused the break can be tagged with breaking-change and then it will get re-organized in the automatic changelog generation.