Add an extra step to the user guide to ensure full Liberty setup in relation to files

[paulbarr]

Hey. I followed this guide: https://github.com/OpenLiberty/liberty-tools-vscode/blob/HEAD/docs/user-guide.md
It's good

However, I found that with my own application I had some issues dealing with files
This was because getRealPath() was returning null for me
It turns out that all I needed to do was to create this directory

/src/main/webapp

In practice most people will have a web.xml file as well, so I suggest that either the plugin auto generates web.xml if it is missing or in the steps that we recommend you create a web.xml file in this location: /src/main/webapp/WEB-INF/web.xml, or say you need to copy your existing web.xml there

I think this should come right after the part where you say where to put the server.xml file

If this sounds good I'm happy to make a PR for this