New reviewer responder docs (#7)

docs/available_responders.md

@@ -39,4 +39,5 @@ Buffy includes a list of Responders that can be used by configuring them in the
    :caption: Custom responders
    :maxdepth: 1
 
+   responders/ropensci/reviewers_due_date
 ```

docs/responders/ropensci/reviewers_due_date.md

@@ -0,0 +1,90 @@
+ROpenSci :: Reviewers & due date
+================================
+
+This responder can be used to add/remove a user to/from the reviewers list in the body of the issue. It also sets a due date for the review and updates that info in the body of the issue and in the reply comment.
+Allows [labeling](../../labeling), that will take effect when the second reviewer is assigned.
+
+## Listens to
+
+```
+@botname add @username to reviewers
+```
+```
+@botname remove @username from reviewers
+```
+
+## Requirements
+
+The body of the issue should have the a couple of placeholders marked with HTML comments: the _reviewers-list_ and the _due-dates-list_
+
+```html
+<!--reviewers-list-->  <!--end-reviewers-list-->
+<!--due-dates-list-->  <!--end-due-dates-list-->
+```
+
+## Settings key
+
+`ropensci_reviewers`
+
+## Params
+```eval_rst
+:due_date_days: *<Integer>* Optional. The number of days from the moment a reviewer is assigned to the due date for the review. Default value is **21** (three weeks).
+
+:sample_value:  Optional. A sample value string for the username field. It is used for documentation purposes when the :doc:`Help responder <../help>` lists all available responders. Default value is **xxxxx**.
+
+:no_reviewer_text: Optional. The text that will go in the removed reviewer place to state there's no one assigned. Default value is **TBD**.
+
+:add_as_assignee: *<Boolean>* Optional. If true, the new reviewer will be added as assignee to the issue. Default value is **false**.
+
+:add_as_collaborator: *<Boolean>* Optional. If true, the new reviewer it will be added as collaborator to the repo. Default value is **false**.
+```
+
+## Examples
+
+**Simplest case:**
+```yaml
+...
+  responders:
+    ropensci_reviewers:
+...
+```
+
+**With labeling, changing no_reviewer_text, limiting access and only if there's an editor already assigned:**
+```yaml
+...
+  responders:
+    ropensci_reviewers:
+      only:
+        - editors
+      if:
+        role_assigned: editor
+      no_reviewer_text: "Pending"
+      add_labels:
+        - 3/reviewer(s)-assigned
+      remove_labels:
+        - 2/seeking-reviewer(s)
+...
+```
+
+## In action
+
+* **`Initial state:`**
+
+Issue's body with placeholders
+![](../../images/responders/ropensci/ropensci_reviewers_due_date_1.png "ROpenSci :: Reviewers & due date: Initial state")
+
+
+* **`Invocation:`**
+
+Assigns first reviewer
+![](../../images/responders/ropensci/ropensci_reviewers_due_date_2.png "ROpenSci :: Reviewers & due date: first assignment")
+
+* **`Assigning second reviewer applies labeling:`**
+![](../../images/responders/ropensci/ropensci_reviewers_due_date_3.png "ROpenSci :: Reviewers & due date: second reviewer and labeling")
+
+
+* **`Final state:`**
+
+Issue's body with reviewers and due dates info
+![](../../images/responders/ropensci/ropensci_reviewers_due_date_4.png "ROpenSci :: Reviewers & due date: Final state")
+