[sr-dev] README-MODULES intro (please review)

SIP-ROUTER modules ================== This is the first release of the combined project. In order to be able to support an upgrade path for both Kamailio/OpenSER and SER current users, we have been forced to provide some modules in two versions, which means that we have three set of modules: - modules: the SIP router modules - modules_k: Kamailio modules - modules_s: SER modules All modules are compatible with the core, so regardless if you are a new user or an existing, you are free to make a choice between the three sets. Note that if you are currently using one product, you want to select the module that exists in the K/S directories first to get a smooth upgrade path. As sip-router development progress, we will move as many modules as possible into the core and only keep k/s modules for backwards compatibility when required.