Documentation: add implementation faq

Add some notes on the design discussion around the `--initial` flags. If
anything is wrong let me know.

Documentation/implementation-faq.md

@@ -0,0 +1,65 @@
+# FAQ
+
+## Initial Bootstrapping UX
+
+etcd initial bootstrapping is done via command line flags such as
+`--initial-cluster` or `--discovery`. These flags can safely be left on the
+command line after your cluster is running but they will be ignored if you have
+a non-empty data dir. So, why did we decide to have this sort of odd UX?
+
+One of the design goals of etcd is easy bringup of clusters using a one-shot
+static configuration like AWS Cloud Formation, PXE booting, etc. Essentially we
+want to describe several virtual machines and bring them all up at once into an
+etcd cluster.
+
+To achieve this sort of hands-free cluster bootstrap we had two other options:
+
+**API to bootstrap**
+
+This is problematic because it cannot be coordinated from a single service file
+and we didn't want to have the etcd socket listening but unresponsive to
+clients for an unbound period of time.
+
+It would look something like this:
+
+```
+ExecStart=/usr/bin/etcd
+ExecStartPost/usr/bin/etcd init localhost:2379 --cluster=
+```
+
+**etcd init subcommand**
+
+```
+etcd init --cluster='default=http://localhost:2380,default=http://localhost:7001'...
+etcd init --discovery https://discovery-example.etcd.io/193e4
+```
+
+Then after running an init step you would execute `etcd`. This however
+introduced problems: we now have to define a hand-off protocol between the etcd
+init process and the etcd binary itself. This is hard to coordinate in a single
+service file such as:
+
+```
+ExecStartPre=/usr/bin/etcd init --cluster=....
+ExecStart=/usr/bin/etcd
+```
+
+There are several error cases:
+
+0) Init has already ran and the data directory is already configured
+1) Discovery fails because of network timeout, etc
+2) Discovery fails because the cluster is already full and etcd needs to fall back to proxy
+3) Static cluster configuration fails because of conflict, misconfiguration or timeout
+
+In hindsight we could have made this work by doing:
+
+```
+rc	status
+0	Init already ran
+1	Discovery fails on network timeout, etc
+0	Discovery fails for cluster full, coordinate via proxy state file
+1	Static cluster configuration failed
+```
+
+Perhaps we can add the init command in a future version and deprecate if the UX
+continues to confuse people.