[dub-guide/selections] Basic content

[WebFreak001]

• describe what it is (it's used for making sure builds and releases use the same package version everywhere, important for reproducible builds)
• tell that it can be edited to force specific package versions
• mention dub upgrade and that it may override user-selection
• mention that dub build puts missing versions into it, same behavior as dub upgrade --missing-only
• tell that it should be checked into version control for applications, that there is debate about whether to put it in libraries - link to https://lsferreira.net/posts/lockfile-lib-misconception/