chore(branding) added branding, best practice to repo

SECURITY.md

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+# Security Policy
+
+## Supported versions
+
+Fixes land on the latest release. Please reproduce on the most recent version before reporting.
+
+## Reporting a vulnerability
+
+**Do not open a public issue.** Report privately through GitHub's
+[private vulnerability reporting](https://github.com/narrrl/proton-drive-linux/security/advisories/new),
+or by email to <contact@narl.io>.
+
+Please include the version (`pdfs --version`), your distribution and desktop environment, what an
+attacker gains, and a reproduction if you have one. You can expect an acknowledgement within a few
+days; a fix ships in the next release, and you are credited in the advisory unless you ask
+otherwise.
+
+## Scope
+
+This client holds Proton account credentials and decrypted file content on the local machine, so
+the interesting boundaries are:
+
+- **Credentials.** Sessions and the mailbox password live in the system Secret Service. Anything
+  that writes them elsewhere — logs, temp files, crash dumps — is a vulnerability.
+- **The control socket.** Anything that can connect to `control.sock` has the daemon's
+  authenticated session. It is `0600`, and the state, cache, and config directories must be real,
+  current-user-owned `0700` directories; the daemon fails closed otherwise. Reports that defeat
+  those checks are in scope.
+- **Local cache and staging.** Decrypted block content and staged writes on disk.
+- **Cryptography.** Key derivation, signature handling, and anything that could publish plaintext
+  or the wrong key material to Proton.
+
+Out of scope: vulnerabilities in Proton's own service (report those to Proton AG), and issues that
+require an attacker who already has your user account on the machine.