Add GUI solo mining guide

I believe I made this guide correctly. This merge request contain the following changes:

1. A new GUI solo mining user guide
2. A link to the guide on the user guides page
3. Several images in the PNG folder that are referenced
4. An update to the prove payment user guide to include information about how to get the transaction key in the GUI

This is my first time using Git desktop, so apologies if I screwed this all up.

resources/user-guides/index.md

@@ -67,7 +67,7 @@ title: "User Guides"
                 </div>
 <div class="row start-xs text-center" markdown="1">
 
-User Guides for Mining coming soon.
+[How to solo mine with the GUI](solo_mine_GUI.html)
 
 </div>
             </div>

resources/user-guides/prove-payment.md

@@ -55,6 +55,8 @@ The information supplied by Alice plugs neatly instead of the placeholders. mone
 key to decode the transaction, and display how much this particular transaction sent to this address. Obviously,
 Charlie will want to double check with Bob the address is really his - same as with Bitcoin.
 
+Alternatively, the transaction key can be obtained in the GUI in the History tab. Click on details for each individual transaction to get the key.
+
 Note: if several transactions were made, this needs repeating for each such transaction.
 
 

resources/user-guides/solo_mine_GUI.md

@@ -0,0 +1,26 @@
+---
+layout: static_page
+title: "How to solo mine with the GUI"
+title-pre-kick: "Solo mining with the GUI"
+title-kick: ""
+title-post-kick: ""
+kick-class: ""
+icon: "icon_userguides"
+attribution: "<!-- Icon is based on work by Freepik (http://www.freepik.com) and is licensed under Creative Commons BY 3.0 -->"
+---
+
+It is very easy to solo mine with the official GUI. If you have not done so already, go to the <a href="/downloads/">Monero downloads page</a> and download the official GUI for your operating system. Then, run the setup and be patient as Monero synchronizes with the network. You should see that it displays "Connected" in the lower left corner.
+
+<img src="/user-guides/png/solo_mine_GUI/01.png" style="width: 600px;"/>
+
+Click on the "Advanced" tab. You should see that several other options appear. Now click on the "Mining" sub-tab.
+
+<img src="/user-guides/png/solo_mine_GUI/02.png" style="width: 600px;"/>
+
+You should now have an option to start mining. You can optionally change the number of threads to mine with. For optimal efficiency, you should mine with your CPU's cache divided by 2. You will need to look up your CPU's specs on the manufacturer's website. If you are unsure, leave the number of threads at 1. Click the "Start mining" button.
+
+<img src="/user-guides/png/solo_mine_GUI/03.png" style="width: 600px;"/>
+
+You are now mining with the network, as you can see on the bottom of the image. In this example, the computer is contributing 23 H/s to the Monero network. Mining helps keep the network secure, and you may get lucky and receive a reward for protecting the network.
+
+To stop mining, simply click the "Stop mining" button.