README: add blockchain-based issue to "Known Issues"

Referencing https://hackerone.com/reports/417515

README.md

@@ -849,7 +849,17 @@ These records are dumped as hex data, where the first line is the key and the se
 
 # Known Issues
 
+## Protocols
+
+### Socket-based
+
 Because of the nature of the socket-based protocols that drive monero, certain protocol weaknesses are somewhat unavoidable at this time. While these weaknesses can theoretically be fully mitigated, the effort required (the means) may not justify the ends. As such, please consider taking the following precautions if you are a monero node operator:
 
 - Run `monerod` on a "secured" machine. If operational security is not your forte, at a very minimum, have a dedicated a computer running `monerod` and **do not** browse the web, use email clients, or use any other potentially harmful apps on your `monerod` machine. **Do not click links or load URL/MUA content on the same machine**. Doing so may potentially exploit weaknesses in commands which accept "localhost" and "127.0.0.1".
 - If you plan on hosting a public "remote" node, start `monerod` with `--restricted-rpc`. This is a must.
+
+### Blockchain-based
+
+Certain blockchain "features" can be considered "bugs" if misused correctly. Consequently, please consider the following:
+
+- When receiving monero, be aware that it may be locked for an arbitrary time if the sender elected to, preventing you from spending that monero until the lock time expires. You may want to hold off acting upon such a transaction until the unlock time lapses. To get a sense of that time, you can consider the remaining blocktime until unlock as seen in the `show_transfers` command.