Update docs - 09-swaps.md (#263)

* Update 09-swaps.md

* Fix typos in 09-swaps.md and add images

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-<!-- TODO: explain and show (UI) how to do P2P LN<-> onchain swaps -->
+Apart from the various fiat payment methods available, there are something
+called *Swap Destinations* as well. These are payment methods, but for BTC, on
+a network other than the Lightning Network. This is useful if you want to
+exchange lightning satoshis for on-chain satoshis (or on any other network like
+Liquid BTC if you prefer). This process of exchanging satoshis on the Lightning
+Network for on-chain satoshis is usually referred to as a "swap".
 
-{% include wip %}
+The table below makes it simple to understand swap in terms of "buyer" and "seller":
+
+| Side   | Sends         | Receives     | Swap type |
+|--------|---------------|--------------|-----------|
+| Seller | ⚡BTC         | 🔗 BTC       | Swap out  |
+| Buyer  | 🔗 BTC        | ⚡BTC        | Swap in   |
+
+
+### How to do a P2P Swap
+
+Remember, in RoboSats you always buy or sell Lightning Network satoshis. If you
+want to receive satoshis over the Lightning Network in exchange for your
+on-chain satoshis, then you create a **BUY** order. On the contrary, if you
+want to receive satoshis on-chain in exchange for your Lightning Network
+satoshis, then create a *SELL* order.
+
+In the Order Create screen, select "BTC" from the currency dropdown menu:
+
+<div align="center">
+    <img src="/assets/images/understand/btc-swap-in-dropdown.png"/>
+</div>
+
+Select your Swap Destination from the dropdown:
+
+<div align="center">
+    <img src="/assets/images/understand/swap-destination-selection.png"/>
+</div>
+
+You then set the amount or range you want to swap for. Remember that if you
+are a seller, then you will receive on-chain BTC and if you are the buyer, you
+will be sending on-chain BTC:
+
+<div align="center">
+    <img src="/assets/images/understand/amount-swap.png"/>
+</div>
+
+You then simply create the order and wait for a taker to take your order. In
+the chatroom you move forward as usual, but this time the payment method is
+simply an on-chain bitcoin address.
+
+
+### Order amount and mining fees
+
+The amount to be sent on-chain must be the exact value mentioned in the order
+amount. The sender of on-chain sats needs to cover for mining fees (on-chain
+transaction fees).
+
+### What premium do I set?
+
+In case of a swap it's better to keep premium to 0% but if you want to make the
+offer a little more attractive to your counterparty you can follow the below
+recommendations:
+
+1. If you are the **seller** - you will be **receiving** on-chain BTC; setting
+   the premium slightly below 0% (eg. -0.1%, -0.5%) will make your offer
+   attractive. The taker already pays 0.125% fees on the trade plus they have
+   to pay mining fees for sending the on-chain BTC.
+
+2. If you are the **buyer** - you will be **sending** on-chain BTC; setting the
+   premium slightly above 0% (eg. 0.1%, 0.5%) will make your offer attractive.
+
+These are just general recommendations about what premium to set to get started
+with swaps, but at the end of the day, the price is what the market sets, so
+experiment and see what works for you.