Add a general overview to the README

README.md

@@ -1,3 +1,46 @@
 # PSRT GHSA Bot
 
-Bot which adds the PSRT GitHub team (`python/psrt`) and CVE IDs to GitHub Security Advisories.
+PSRT GHSA Bot is a GitHub App that automates the [Python Security Response Team
+(PSRT)](https://devguide.python.org/security/psrt/)'s
+handling of GitHub Security Advisories. It runs hourly (or by manual dispatch)
+and, for every advisory it closes ones marked as completed, promotes accepted ones
+from triage to draft, reserves CVE IDs, creates private forks, and adds the
+PSRT members as collaborators.
+
+```mermaid
+flowchart TD
+    Start([Hourly cron or manual dispatch]):::entry --> Advs[For each repository security advisory]:::loop
+    Advs --> S{"Advisory state?"}:::decision
+
+    S -- Closed --> Skip([Skip advisory]):::terminal
+    S -- Triage --> Collab
+    S -- Draft --> Collab
+
+    Collab{"PSRT missing as collaborators?"}:::decision
+    Collab -- yes --> AddCollab[Add PSRT as collaborators]:::write --> Tag
+    Collab -- no --> Tag
+
+    Tag{"Summary has a completion tag?"}:::decision
+    Tag -- yes --> Close[Close advisory]:::write --> Done([Continue to next advisory]):::terminal
+    Tag -- "No, Triage" --> Accept{"Summary has an accept tag?"}:::decision
+    Tag -- "No, Draft" --> Fork
+
+    Accept -- yes --> ToDraft[Move to the draft state]:::write --> Fork
+    Accept -- no --> Update
+
+    Fork{"No private fork?"}:::decision
+    Fork -- yes --> MkFork[Create private fork]:::write --> Cve
+    Fork -- no --> Cve
+
+    Cve{"No CVE ID assigned?"}:::decision
+    Cve -- yes --> Reserve[Reserve a CVE ID]:::write --> Update
+    Cve -- no --> Update
+
+    Update[Update advisory]:::write --> Done
+
+classDef entry stroke:#0C0,stroke-width:2px;
+classDef loop stroke:#00C;
+classDef decision stroke:#CC0;
+classDef write stroke:#C0C;
+classDef terminal stroke:#0C0;
+```