Add user documentation for managing large changesets

Summary: See task and P91, this just adapts my Discussion post to be more
general. I'll follow up by linking to it from the Arcanist error message.
Test Plan: Read the document.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 701

src/docs/userguide/differential_large_changes.diviner

@@ -0,0 +1,54 @@
+@title Differential User Guide: Large Changes
+@group userguide
+
+Dealing with huge changesets, and when **not** to use Differential.
+
+= Overview =
+
+When you want code review for a given changeset, Differential is not always the
+right tool to use. The rule of thumb is that you should only send changes to
+Differential if you expect humans to review the actual differences in the source
+code from the web interface. This should cover the vast majority of changes but,
+for example, you usually should //not// submit changes like these through
+Differential:
+
+  - Committing an entire open source project to a private repo somewhere so
+    you can fork it or link against it.
+  - Committing an enormous text datafile, like a list of every English word or a
+    dump of a database.
+  - Making a trivial (e.g., find/replace) edit to 10,000 files.
+
+You can still try submitting these kinds of changes, but you may encounter
+problems getting them to work (database or connection timeouts, for example).
+Differential is pretty fast and scalable, but at some point either it or the
+browser will break down: you simply can't show nine million files on a webpage.
+
+More importantly, in all these cases, the text of the changes won't be reviewed
+by a human. The metadata associated with the change is what needs review (e.g.,
+what are you checking in, where are you putting it, and why? Does the change
+make sense? In the case of automated transformations, what script did you use?).
+To get review for these types of changes, one of these strategies will usually
+work better than trying to get the entire change into Differential:
+
+  - Send an email/AIM/IRC to your reviewer(s) like "Hey, I'm going to check in
+    the source for MySQL 9.3.1 to /full/path/to/whatever. The change is staged
+    in /home/whatever/path/somewhere if you want to take a look. Can I put your
+    name on the review?". This is best for straightforward changes. The reviewer
+    is not going to review MySQL's source itself, instead they are reviewing the
+    change metadata: which version are you checking in, why are you checking it
+    in, and where are you putting it?
+  - Create a Differential revision with only the metadata, like the script you
+    used to make automated changes or a text file explaining what you're doing,
+    and maybe a sample of some of the changes if they were automated. Include a
+    link to where the changes are staged so reviewers can look at the actual
+    changeset if they want to. This is best for more complicated changes, since
+    Differential can still be used for discussion and provide a permanent record
+    others can refer to.
+
+In both cases, you won't be able to use "arc commit" or "arc amend" to actually
+push the change. Just use "svn" or "git" and manually edit the commit message
+instead. (It is normally sufficient to add a "Reviewed By: <username>" field.)
+
+These kinds of changes are generally rare and don't have much in common, which
+is why there's no explicit support for them in Differential. If you frequently
+run into cases which Differential doesn't handle, let us know what they are.