Add some more docs: simplegraphic api, native busted

Author: vaisest

CONTRIBUTING.md

@@ -138,6 +138,12 @@ such as [VSCodium](https://vscodium.com) or [Eclipse Theia](https://theia-ide.or
 
 Another alternative on VSCode is to use [sumneko's Lua language server](https://marketplace.visualstudio.com/items?itemName=sumneko.lua) along with [actboy168's debugger](https://marketplace.visualstudio.com/items?itemName=actboy168.lua-debug). These can potentially offer more features than EmmyLua, such as conditional breakpoints.
 
+## Runtime environment
+
+It is recommended that you configure your IDE to include `src/_SimpleGraphic.def.lua` somehow. Path of Building runs inside a special Lua environment via SimpleGraphic which implements a small API. These are not defined inside the project, which means that the aforementioned meta/hint file is required for the IDE to know which functions exist. As the file is not included in code, it must be explicitly mentioned as a library in whichever language server you are using, or otherwise it will not be read.
+
+This file is normally not executed, but does contain basic implementations for parts of the API, which allows many parts of PoB to work without running inside SimpleGraphic. If you wish to test individual changes, it might be possible to do so through a script using Luajit directly. To do so, see HeadlessWrapper.lua for an example. It should be noted that some parts of the API (such as subscripts) are not implemented, which means some parts of PoB are unusable.
+
 ### Visual Studio Code
 
 1. Create a new <kbd>Debug Configuration</kbd> of type <kbd>EmmyLua New Debug</kbd>
@@ -244,6 +250,9 @@ If you prefer to not use EmmyLua, the following configuration works well for Sum
 
         // this has lua 5.3 code which produces errors, but doesn't actually run
         "src/runtime/lua/sha1/*",
+
+        // avoid overriding the below library setting
+        "src/_SimpleGraphic.def.lua",
     ],
     "Lua.diagnostics.disable": ["inject-field"],
     // disables diagnostics even when you open one of the above
@@ -296,6 +305,13 @@ More tests can be added to this folder to test specific functionality, or new te
 
 Please try to include tests for your new features in your pull request. Additionally, if your pr breaks a test that should be passing please update it accordingly.
 
+It can also be a good idea to install `busted` locally as it tends to be faster to execute:
+
+1. Install Luajit (due to PoB accessing the jit library, it non-Luajit versions might not work)
+2. Install [Luarocks](https://luarocks.org/) (for example, through Scoop or your Linux package manager)
+3. Run `luarocks install busted`
+4. Run `busted --lua=luajit` to run the tests. You can also use e.g. `-p TestUtils_spec.lua` to run a specific file.
+
 ### Debugging tests
 When running tests with a docker container it is possible to use EmmyLua for debugging. Paste in the following right under `function launch:OnInit()` in `./src/Launch.lua`:
 ```lua