Indicate where git-annex will lead to pantry error

doc/pantry.md

@@ -214,6 +214,26 @@ extra-deps:
   commit: 2f8a8e1b771829f4a8a77c0111352ce45a14c30f
 ```
 
+#### Limited [git-annex](https://git-annex.branchable.com) support
+
+Pantry does not support [git-annex](https://git-annex.branchable.com). This is
+because `git archive` does not handle symbolic links outside the work tree. It
+is still possible to use repositories which use git-annex but do not require the
+annex files for the package to be built.
+
+To do so, ensure that any files or directories stored by git-annex are marked
+[export-ignore](https://git-scm.com/docs/git-archive#Documentation/git-archive.txt-export-ignore)
+in the `.gitattributes` file in the repository. See
+[#4579](https://github.com/commercialhaskell/stack/issues/4579) for more
+information.
+
+For example, if the directory `fonts/` is controlled by git-annex, use the
+following line.
+
+```gitattributes
+fonts export-ignore
+```
+
 ### Archives (HTTP(S) or local filepath)
 
 You can use HTTP and HTTPS URLs and local filepaths referring to

subs/pantry/src/Pantry/Archive.hs

@@ -394,7 +394,9 @@ parseArchive rpli archive fp = do
                   ]
                 Right x -> Right x
             case Map.lookup dest files of
-              Nothing -> Left $ "Symbolic link dest not found from " ++ mePath me ++ " to " ++ relDest ++ ", looking for " ++ dest
+              Nothing -> Left $ "Symbolic link dest not found from " ++ mePath me ++ " to " ++ relDest ++ ", looking for " ++ dest ++ ".\n"
+                  ++ "This may indicate that the source is a git archive which uses git-annex.\n"
+                  ++ "See https://github.com/commercialhaskell/stack/issues/4579 for further information."
               Just me' ->
                 case meType me' of
                   METNormal -> Right $ SimpleEntry dest FTNormal