refactor: Update database schema for encrypted columns

This commit updates the README.md file to include an example of the database schema for storing encrypted data. It specifies that all columns intended for storing encrypted data must be of type `TEXT` in the database. The example provided demonstrates the creation of a table named "test" with two columns: "string" and "numbers", both of type `TEXT`.

This change improves the clarity and consistency of the documentation regarding the database schema for encrypted columns.

README.md

@@ -42,3 +42,13 @@ session.execute(
     sql"SELECT * FROM test".query(crypt.text ~ crypt.int4)
 )
 ```
+
+All columns intended for storing encrypted data must be of type `TEXT` in the database.
+
+e.g.:
+```sql
+CREATE TABLE test (
+    string TEXT,
+    numbers TEXT
+)
+```