readme: added deltachat "no valid signatures" error src ref

README.md

@@ -0,0 +1,10 @@
+## known issues
+
+### deltachat
+
+```
+src/e2ee.rs:282: Detected Autocrypt-mime message
+src/mimeparser.rs:166: decryption failed: "no valid signatures"
+```
+
+https://github.com/deltachat/deltachat-core-rust/blob/3a25d6b275eb2d33a64dbc7a60e37df2bf18a559/src/e2ee.rs#L309