doc: update bitcoin-conf.md

include instructions on how to run the script

Author: josibake

doc/bitcoin-conf.md

@@ -61,4 +61,12 @@ Windows | `%APPDATA%\Bitcoin\` | `C:\Users\username\AppData\Roaming\Bitcoin\bitc
 Linux | `$HOME/.bitcoin/` | `/home/username/.bitcoin/bitcoin.conf`
 macOS | `$HOME/Library/Application Support/Bitcoin/` | `/Users/username/Library/Application Support/Bitcoin/bitcoin.conf`
 
-You can find an example bitcoin.conf file in [share/examples/bitcoin.conf](../share/examples/bitcoin.conf).
+An example configuration file can be generated by [contrib/devtools/gen-bitcoin-conf.sh](../contrib/devtools/gen-bitcoin-conf.sh).
+Run this script after compiling to generate an up-to-date configuration file.
+The output is placed under `share/examples/bitcoin.conf`.
+To use the generated configuration file, copy the example file into your data directory and edit it there, like so:
+
+```
+# example copy command for linux user
+cp share/examples/bitcoin.conf ~/.bitcoin
+```