adding common-questions doc page

Author: Shlomi Noach

doc/questions.md

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+# How?
+
+### How does the cut-over work? Is it really atomic?
+
+The cut-over phase, where the original table is swapped away, and the _ghost_ table takes its place, is an atomic, blocking, controlled operation.
+
+- Atomic: the tables are swapped together. There is no gap where your table does not exist.
+- Blocking: all app queries involving the migrated (original) table are either operate on the original table, or are blocked, or proceed to operate on the _new_ table (formerly the _ghost_ table, now swapped in).
+- Controlled: the cut-over times out at pre-defined threshold, and is atomically aborted, then re-attempted. Cut-over only takes place when no lags are present, and otherwise no throttling reason is found. Cut-over step itself gets high priority and is never throttled.
+
+Read more on [cut-over](cut-over.md) and on the [cut-over design Issue](https://github.com/github/gh-ost/issues/82)
+
+
+# Is it possible to?
+
+### Is it possible to add a UNIQUE KEY?
+
+Adding a `UNIQUE KEY` is possible, in the condition that no violation will occur. That is, you must make sure there aren't any violating rows on your table before, and during the migration.
+
+At this time there is no equivalent to `ALETER IGNORE`, where duplicates are implicitly and silently thrown away. This behavior may be supported in the future.
+
+# Why