Create entity relationship diagram

docs/entity_relationship_diagram.md

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+# PERS Database Entity Relationship Diagram 
+
+The Publication Email Request Service (PERS) relies on a database to keep track of the email requests that have been sent by stakeholders. Monitoring the email requests via database prevents duplication of email requests. The diagram describes the relationships between entities, or tables, in the database.
+
+The relationships depicted in the diagram can be summarized as follows:
+* A `Liaison` belongs to **one** `DLC`.
+* A `DLC` is assigned to **one or more** authors.
+* A `Publication (needing request)` can be associated with **one or more** `Author(s)`.
+* A `Publication (needing request)` can only be included in **one** `Email`; an `Email` can include **one or more** `Publication(s)`.
+* When an `Email` is created, it is assigned to **one** `Author` and **one** `Liaison`.
+
+```mermaid
+erDiagram
+    publication["Publication (needing request)"]{
+        string id PK 
+        string title
+        string citation
+        string email_id FK
+    }
+    dlc[DLC]{
+        string id PK
+        string name
+    }
+    liaison[Liaison]{
+        string id PK
+        string first_name
+        string last_name
+        string email_address
+        bool active
+    }
+    author[Author]{
+        string id PK
+        string dlc FK
+        string email_address
+        string first_name
+        string last_name
+    }
+    email[Email]{
+        string id PK
+        richtext original_text
+        richtext latest_text
+        date date_sent
+    }
+    liaison }o--o| dlc: "belongs to"
+    dlc ||--|{ author: has
+    author }|--o{ publication: has
+    publication }o--|| email: "included in"
+    author ||--o{ email: receives
+    liaison ||--o{ email: receives
+```