Add next_stage spec (#3409)

Add README to next_stage module.

modules/next_stage/README.md

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+# Next Stage
+
+## Purpose
+
+The `next_stage` module provides a method to start the "next stage"
+of a candidate session. The "Stage" of a session goes from Screening
+to Visit to Approval. The Screening and Visit stages may have
+different instruments administered. The `next_stage` module prompts
+for the date of the visit and uses the data it gathers to populate
+the test battery for the Visit stage.
+
+However, note that the Screening stage is mostly historic and poorly
+tested.  Most LORIS studies prefer to have a separate "Screening"
+visit, rather than a screening stage for each visit. The result is
+that the `next_stage` module is mostly used simply to populate the
+test battery for a session.
+
+## Intended Users
+
+The `next_stage` module is used by data entry staff in order to
+start the Visit stage of an instrument.
+
+## Scope
+
+The `next_stage` module only moves the stage of a session from the
+Screening to Visit. The final Approval or Recycling Bin stage is
+reached by sending to DCC (or Recycling Bin) on the `instrument_list`
+page.
+
+## Permissions
+
+The `data_entry` permission is required. The user must also be at
+the same CenterID as the timepoint being accessed.
+
+Access is also denied if the timepoint isn't startable (ie. if it's
+already been started.)
+
+## Configurations
+
+The `test_battery` table must be set up with instruments to be added
+to the `Visit` stage which meets the criteria of the timepoint (ie
+age range, visit, center..) or no tests will be added. This must
+be done before the stage is started.
+
+## Interactions with LORIS
+
+The `next_stage` module links back to the `instrument_list` page
+after a stage is started.
+
+The `next_stage` module inserts the on-site test battery into the
+flag table. It does NOT insert survey instruments, which are inserted
+by the `survey_accounts` module.