Add Storing Emails With citext as a postgres til

postgres/storing-emails-with-citext.md

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+# Storing Emails With citext
+
+Email addresses should be treated as case-insensitive because they are. If a
+user is trying to sign in with their email address, we shouldn't care if
+they type `user@example.com` or `User@example.com`. Both of those email
+addresses should be treated as equal and ultimately lead us to the same
+`User` record.
+
+With the
+[`citext`](http://www.postgresql.org/docs/current/static/citext.html)
+extension, we can create a column that acts as a case-insensitive text type.
+Any comparisons on a column of that type will internally have the `lower`
+function executed on the arguments.
+
+The following example shows this in action:
+
+```sql
+create extension if not exists citext;
+
+create table citext_emails (
+  id serial primary key,
+  email citext not null unique
+);
+
+insert into citext_emails (email) values ('LizLemon@nbc.com');
+
+select * from citext_emails where email = 'lizlemon@nbc.com';
+--  id |      email
+-- ----+------------------
+--   1 | LizLemon@nbc.com
+```
+
+See
+[`citext-emails.sql`](https://github.com/jbranchaud/postgresing/blob/master/citext-emails.sql)
+for a full example.