Add section on external dependencies

[mrcjkb]

Example:

sqlite.lua depends on libsqlite (e.g. libsqlite3.so).
It requires the user to set g:sql_clib_path so that it can find it.

This approach has the following problems:

• Declaring dependencies is the user's responsibility (see why rocks.nvim).
• Or it becomes the package manager's responsibility.

Both cases are prone to instability.

Instead, plugins should auto-detect external dependencies (perhaps Neovim should provide an API for this? 🤔)
and fall back to a user config or an environment variable if the dependency is not found.

If a plugin is published to luarocks, its external dependencies should be specified in the rockspec.
See for example fugit2.nvim.