Add orderbook.md doc file

docs/orderbook.md

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+# Orderbook
+
+![](images/jm_orderbook.png)
+
+To setup your own local orderbook, the only thing you need is an install of Joinmarket (you don't need Bitcoin Core for that);
+follow either the Quick Install on the readme (`./install.sh`) (RECOMMENDED), or one of the manual installation as per [here](INSTALL.md).
+
+## Configuration
+
+To run the orderbook without access to Bitcoin Core, edit the `joinmarket.cfg` file in your Joinmarket data directory as per below
+(NOTE: if you don't have a data directory, it will be created the first time you run the script).
+
+```
+[BLOCKCHAIN]
+blockchain_source = no-blockchain
+```
+
+Comments are omitted; do read them.
+
+You may also want to check the `[MESSAGING:server]` options, e.g., if you want to use IRC Tor onion services you can edit the relevant parts.
+
+## Start
+
+Launch the `ob-watcher.py` script from `scripts/obwatch` as any other Joinmarket script.
+
+`(jmvenv)$ python ob-watcher.py`
+
+Add `-p PORT` to use a custom port (default is equivalent to `-p 62601`).
+
+Then open the link to the just created local server in your favorite browser.