add docs for Milestones API

doc/api/milestones.md

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+## List project milestones
+
+Get a list of project milestones.
+
+```
+GET /projects/:id/milestones
+```
+
+Parameters:
+
++ `id` (required) - The ID or code name of a project
+
+## Single milestone
+
+Get a single project milestone.
+
+```
+GET /projects/:id/milestones/:milestone_id
+```
+
+Parameters:
+
++ `id` (required) - The ID or code name of a project
++ `milestone_id` (required) - The ID of a project milestone
+
+## New milestone
+
+Create a new project milestone.
+
+```
+POST /projects/:id/milestones
+```
+
+Parameters:
+
++ `id` (required) - The ID or code name of a project
++ `title` (required) - The title of an milestone
++ `description` (optional) - The description of the milestone
++ `due_date` (optional) - The due date of the milestone
++ `closed` (optional) - The status of the milestone
+
+## Edit milestone
+
+Update an existing project milestone.
+
+```
+PUT /projects/:id/milestones/:milestone_id
+```
+
+Parameters:
+
++ `id` (required) - The ID or code name of a project
++ `milestone_id` (required) - The ID of a project milestone
++ `title` (optional) - The title of a milestone
++ `description` (optional) - The description of a milestone
++ `due_date` (optional) - The due date of the milestone
++ `closed` (optional) - The status of the milestone