wip: list known issues in documentation

doc/book/reference-manual/known-issues.md

@@ -0,0 +1,38 @@
++++
+title = "Known issues"
+weight = 80
++++
+
+
+## Architectural limitations
+
+Issues that are caused by design decisions of Garage internals, and that can't
+be fixed without major architectural changes in the codebase.
+
+### Buckets are not sharded
+
+### Very big objects cause performance degradation
+
+### No locking (`if-none-match`, ...)
+
+### `CreateBucket` race condition
+
+
+
+## Bugs
+
+Known bugs that are complex to diagnose and fix, and therefore have not been
+fixed yet.
+
+### LMDB metadata corruption
+
+### Layout updates might require manual intervention
+
+
+
+## General footguns
+
+Choices made by the developers that users must be aware of if they don't want
+to run into potential issues.
+
+### Resync tranquility is conservative by default