Docs: publish an add-on via localtunnel

[Ivshti]

This might be an interesting usecase, if you want to quickly publish an add-on

[jaruba]

An example of using localtunnel is now part of stremio-express-addon-example

Will mention it in the addon sdk docs too as it can also be used with the sdk.

[jaruba]

Notes about using localtunnel also added to the Readme files of: https://github.com/Stremio/addon-helloworld https://github.com/Stremio/addon-helloworld-express

As it's also relevant information there too