Add new documentation for the owners-autoassign plugin

The owners-autoassign plugin shares the same documentation
of the owners plugin. However, since the introduction of
I7269e857e5f, it needs now an optional owners-autoassign.config
file and therefore needs an additional configuration add-on.

Change-Id: I7cfa53115733601fc1d6b7df32402b99ffcfc195

owners-autoassign/src/main/resources/Documentation/auto-assign.md

diff --git a/owners-autoassign/src/main/resources/Documentation/auto-assign.md b/owners-autoassign/src/main/resources/Documentation/auto-assign.md
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+## Reviewers auto-assign configuration
+
+The OWNERS file is processed by the @PLUGIN@ for automatically
+assigning all relevant owners to a change for every new patch-set
+uploaded.
+
+The way that the reviewers are added is controlled by the
+$GERRIT_SITE/etc/@PLUGIN@.config file.
+
+By default, all reviewers are added synchronously when a patch-set
+is uploaded. However, you may want to delay the assignment of additional
+reviewers to a later stage for lowering the pressure on the Git
+repository associated with concurrent updates.
+
+For example, the following configuration would delay the assignment of
+reviewers by 5 seconds:
+
+```
+[reviewers]
+  async = true
+  delay = 5 sec
+```
+
+See below the full list of configuration settings available:
+
+- `reviewers.async`: assign reviewers asynchronously. When set to `false`, all
+  the other settings in @PLUGIN@.config are ignored. By default, set to `false`.
+
+- `reviewers.delay`: delay of the assignment of reviewers since the upload
+  of a new patch-set, expressed in <number> <unit>. By default, set to `0`.
+
+  Values should use common unit suffixes to express their setting:
+
+  - ms, milliseconds
+
+  - s, sec, second, seconds
+
+  - m, min, minute, minutes
+
+   - h, hr, hour, hours
+
+- `reviewers.retryCount`: number of retries for attempting to assign reviewers
+  to a change. By default, set to `2`.
+
+- `reviewers.retryInterval`: delay between retries. Expressed in the same format
+  of the `reviewers.delay`. By default, set to the same value of `reviewers.delay`.
+
+- `reviewers.threads`: maximum concurrency of threads for assigning reviewers to
+  changes. By default, set to 1.
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