README: update attach instructions for testnet users (ethereum#14448)

README.md

@@ -90,15 +90,17 @@ here.
 Specifying the `--testnet` flag however will reconfigure your Geth instance a bit:
 
  * Instead of using the default data directory (`~/.ethereum` on Linux for example), Geth will nest
-   itself one level deeper into a `testnet` subfolder (`~/.ethereum/testnet` on Linux).
+   itself one level deeper into a `testnet` subfolder (`~/.ethereum/testnet` on Linux). Note, on OSX
+   and Linux this also means that attaching to a running testnet node requires the use of a custom
+   endpoint since `geth attach` will try to attach to a production node endpoint by default. E.g.
+   `geth attach <datadir>/testnet/geth.ipc`. Windows users are not affected by this.
  * Instead of connecting the main Ethereum network, the client will connect to the test network,
    which uses different P2P bootnodes, different network IDs and genesis states.
-
+   
 *Note: Although there are some internal protective measures to prevent transactions from crossing
-over between the main network and test network (different starting nonces), you should make sure to
-always use separate accounts for play-money and real-money. Unless you manually move accounts, Geth
-will by default correctly separate the two networks and will not make any accounts available between
-them.*
+over between the main network and test network, you should make sure to always use separate accounts
+for play-money and real-money. Unless you manually move accounts, Geth will by default correctly
+separate the two networks and will not make any accounts available between them.*
 
 #### Docker quick start