Add a specification document

doc/specification.rst

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+Specification
+=============
+
+The goal of rayr is to have various *sources* (local folders) synchronized with *destinations* - or remotes (public or private repositories hosting servers).
+When we talk about synchronization, it is a stateless sync, ala rsync or rclone. The tool has no memory of what the previous synchronization did.
+However, it will do its best to put both ends at the same state, repository wise.
+
+As such, we can base some of the way to interact with rayr as with rsync or rclone.
+
+Repositories format
+-------------------
+
+For now, rayr only deals with git repositories. It should be fairly easy, and desirable, to add mercurial to the list. Other SCMs could be added, provided they have a similar push/pull paradigm and some online servers supports them.
+
+Providers
+---------
+
+The following providers will be supported
+ * Github
+ * Gitlab
+ * Bitbucket
+ * Gogs and Gitea
+
+We possibly could support Rhodecode in the community edition variation.
+
+Configuration format
+--------------------
+
+[This is a proposal and not the current way of configuration]
+Configuration should done in a format that is human readable, such as json, yaml, or config file format.
+Configuration should also be doable with the tool itself, upon invoking ``rayr config``
+
+We should display current remotes and a way to add, modify and delete a remote.
+
+Remotes have short names that allow the user to remember them. In a classic scenario, we can provide reasonable defaults (such as ``github``, ``gitlab`` and so on).
+
+Authentication
+--------------
+
+Most providers provide two ways to authenticate. With OAuth or with application password. As **rayr** is open source, it cannot embed its own set of credentials.
+Therefore, the user must provide them. Alternatively, we can ask the user to provide an application password for the providers that supports them.
+
+Synchronization
+---------------
+
+We can synchronize with the following command:
+
+.. code:: bash
+
+    rayr sync <localfolder> <remotename>
+    rayr sync <remotename> <localfolder>
+
+
+Options
+-------
+
+We should support classic options: log location, verbosity, config file location override.