[CF 5771] v1 - Add README for catalog

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Patch(es): https://www.postgresql.org/message-id/8BEEFD99-E003-460B-8141-6EBC08BAB116@gmail.com
Author(s): Florents Tselai

Author: Commitfest Bot

src/include/catalog/README

@@ -0,0 +1,11 @@
+# Catalog
+
+For more details see https://www.postgresql.org/docs/current/bki.html
+
+Below are some checklists for common scenarios.
+
+## Adding a New Built-in SQL Command
+
+1. Implement the function in C and place it in the appropriate directory under `src` (typically in `src/utils/adt`)
+2. Each function should have a unique integer OID. Run the script `src/include/catalog/unused_oids` to find available ranges.
+3. Add the entry to `pg_proc.dat` following the surrounding conventions.