[IMP] company_today: Document raison d'être in description

I'm drafting an e-mail to the OCA contributors list with a description
of this module, and I thought I'd backport it to the repository.

Signed-off-by: Carmen Bianca Bakker <[email protected]>

Author: carmenbianca

company_today/readme/DESCRIPTION.rst

@@ -1,2 +1,24 @@
 Store today's date on the company module. A cronjob makes sure this field stays
 up-to-date.
+
+The use-case for this module is as follows. Imagine you have two regular fields
+and a computed field::
+
+    amount = fields.Monetary()
+    payment_date = fields.Date()
+    cost_per_day = fields.Monetary(compute="_compute_cost_per_day")
+
+    @api.depends("amount", "payment_date")
+    def _compute_cost_per_day(self):
+        today = fields.Date.today()
+        for record in self:
+            delta = today - record.payment_date
+            record.cost_per_day = record.amount / delta.days
+
+When the next day arrives, ``cost_per_day`` should be re-computed, but it won't
+be, because none of the dependencies have changed.
+
+With this module, you can add a dependency on "company_id.today" (assuming that
+your model has a res.company field, which is trivial to add). This way, you get
+all the benefits of having a compute cache, and your field will be recomputed
+when the date changes.