[ADD] contract_price_revision: new module to update contract prices easily

contract_price_revision/readme/CONTRIBUTORS.rst

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+* `Tecnativa <https://www.tecnativa.com>`_:
+
+  * Vicent Cubells

contract_price_revision/readme/DESCRIPTION.rst

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+This module allows to create revisions of contract line prices.

contract_price_revision/readme/USAGE.rst

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+To use this module:
+
+#. Go to **Invoicing > Documents > Customer Contracts** if you are a billing
+   user.
+#. Select as many contracts as you want to update line prices.
+#. Click on Action button and execute the wizard **Create revision of contract
+   lines**.
+#. Enter date start from which the new price will be valid and enter date
+   end and percentage to increase old contract lines.
+#. By clicking on Apply button, a new contract line will be created with
+   a price increased accordingly to the percent entered. Old contract lines
+   will have as ending date the day before the entered date.
+#. When create invoices related to contracts selected, prices will be update
+   with the prices that are not out of date.