Adds the first draft for the documentation of how to port a feature from EE to CE

doc/development/porting_a_feature_to_ce.md

+# Porting a feature from Enterprise Edition (EE) to Community Edition (CE)
+
+Porting a feature from one version to the other can sometimes be a complicated process
+with many pitfalls, this document lays out some lessons learned and common pitfalls to avoid.
+
+## The EE counterpart should always be the superset of all changes
+
+There should be no change in the CE version that is not in EE.
+
+If a table being ported has extra columns in EE than in CE make sure you add those columns 
+in a separate EE-only migration.
+
+## Scope
+
+You should always try to reduce the scope as much as possible, to keep the changes intuitive
+and easy to review. 
+
+A good example would be if there are a few methods in a model that need to be ported over 
+to another model. In that case, a good possibility would be to begin by delegating every method
+to that model, and then, make a separate merge request that finishes the port of those methods.
+
+## Migrations
+
+Migrations can become quite a complex matter at GitLab's scale so they should always be handled with
+extreme care.
+
+Make sure you read the [What requires downtime guidelines][downtime] and the
+[Background migrations guidelines][background].
+
+If a table needs to be migrated, you should always base yourself on the actual state of the schema 
+rather than looking at each migration related with the table in question.
+
+If we are dealing with a large volume of data that needs to be ported, batching should be considered.
+For more information refer to the [iterating tables in batches guidelines][batches].
+
+Always make sure the database changes are MySQL compatible. 
+
+## Pipelines
+
+The EE specific pipelines, such as `ee_compat_check`, `ee-specific-lines-check` and `ee-files-location-check`
+become especially useful when porting a feature over to CE since they will be able to help you prevent
+possible conflicts when a CE-to-EE merge takes place.
+
+[downtime]: https://docs.gitlab.com/ee/development/what_requires_downtime.html
+[background]: https://docs.gitlab.com/ee/development/background_migrations.html
+[batches]: https://docs.gitlab.com/ce/development/iterating_tables_in_batches.html