Main channel vs vanilla

Continuing the discussion from #166.

## How to determine the set of patches/fixes applied

For tools whose binaries are available in both `main` and `vanilla` channels,
say `HLS 2.5.0.0`, what are the differences in terms of the specific
"patches/fixes" (as stated in the `README.md`) that are applied to `main`? How
would an end-user of `ghcup` determine that?

## Which takes precedence

~~If one were to enable the `vanilla` channel in addition to the `main` via
below, which channel would be given priority when installing tools whose
binaries are available in both channels?~~

``` shell
ghcup config add-release-channel https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-vanilla-0.0.8.yaml
```

Answering the above myself: ^ reading the source of `config.yaml` in `ghcup-hs`,
it's clear that later entries would win. I.e, in the example above `vanilla`
would take precedence over `main`. From `config.yaml` source:

``` yaml
# Where to get GHC/cabal/hls download info/versions from. This is a list that performs
# union over tool versions, preferring the later entries.
```

Thank you to both the upstream and ghcup maintainers for their continued
efforts.

Start with README.md for the description of patches and fixes, then read the referenced config.yaml source in ghcup-hs to verify how channel entries are combined. Document how an end user can identify the differences applied to main and confirm the precedence behavior described in the issue; the issue does not name a target documentation file.

Written by the indexing model from the issue text.