Add a disclaimer about version constraints and Stackage

[artamonovkirill]

At the moment, the Haskell track allows editing the package.yaml.

Yes, it is unclear that any version (using a version constraint) wouldn't work, since a specific version of Stackage is used behind the scenes.

If a version constraint cannot be satisfied, a test run fails with:

No information from Hackage index, updating
Selected mirror https://hackage.haskell.org/
Downloading root
HttpExceptionRequest Request {
  host                 = "hackage.haskell.org"
  port                 = 443
  secure               = True
  requestHeaders       = [("Accept-Encoding",""),("User-Agent","Haskell pantry package")]
  path                 = "/root.json"
  queryString          = ""
  method               = "GET"
  proxy                = Nothing
  rawBody              = False
  redirectCount        = 10
  responseTimeout      = ResponseTimeoutDefault
  requestVersion       = HTTP/1.1
}
 (ConnectionFailure Network.Socket.getAddrInfo (called with preferred socket type/protocol: AddrInfo {addrFlags = [AI_ADDRCONFIG], addrFamily = AF_UNSPEC, addrSocketType = Stream, addrProtocol = 0, addrAddress = <assumed to be undefined>, addrCanonName = <assumed to be undefined>}, host name: Just "hackage.haskell.org", service name: Just "443"): does not exist (Try again))

Exercises in the track are designed to be basic, so it's unlikely that someone might need more than some common functions - e.g., the latest version of some library. A disclaimer like the one below should probably be enough in the package.yaml:

# Do not specify version bounds!
# Versions are derived from the Stackage snapshot.

Even better if those files can be autogenerated with the version of Stackage specified as well.

Check the initial discussion here: #1041