Updates README for documenting the saml_idp_response_url variable

[Robb-Fr]

This documentation update aims to help users solving non working IdP initiated Single Logout due to a potential endpoints difference between the SingleLogoutRequest and SingleLogoutResponse consumption endpoints on the IdP.

Indeed, the saml_idp_response_url variable is used for this purpose in saml_sp.js#L1230 but is never populated by saml_sp_configuration.conf.

I personally first added some code to the NJS script before realizing this step is already done for us, there only needs to be a variable setting.

F5 Big IP APM notably has 2 different endpoints for SingleLogoutRequest and SingleLogoutResponse. This change will ease the integration of NGINX with F5 Big IP APM IdP for example.