test: Add notes on how to generate data/wallets/high_minversion

test/functional/data/wallets/high_minversion/GENERATE.md

@@ -0,0 +1,8 @@
+The wallet has been created by starting Bitcoin Core with the options
+`-regtest -datadir=/tmp -nowallet -walletdir=$(pwd)/test/functional/data/wallets/`.
+
+In the source code, `WalletFeature::FEATURE_LATEST` has been modified to be large, so that the minversion is too high
+for a current build of the wallet.
+
+The wallet has then been created with the RPC `createwallet high_minversion true true`, so that a blank wallet with
+private keys disabled is created.