Add maintenance document

MAINTENANCE.md

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+## Current state
+
+readr is in somewhat of a weird state currently. We have both the older v1 parsing code base, as well as calls to vroom under the hood in v2.
+
+In addition there are still some parts, like the `parse_()` functions which just use the v1 code, as there are not currently analogous APIs in vroom.
+
+## Known outstanding issues
+
+Lazy reading causes problems for people in practice because of file locking on windows.
+This makes the technique less effective then it would otherwise be unfortunately.
+
+The v1 parser has some issues around skipping lines and quoting. There was a bunch of flux in how skipped lines were counted.
+
+For known issues for the v2 parser see the vroom MAINTENANCE.md document.
+
+## Future directions
+
+I think eventually the v1 parsing code should be removed entirely and either
+the vroom codebase should be included directly in readr and vroom should be archived, or
+the two should both continue to exist with the readr functions being a lightweight
+shim over the calls to vroom.