Copy and paste the wiki guide on require

I wrote the original, so I don't need to feel guilty :)

Closes #565.

doc/guides/using_require.md

@@ -0,0 +1,83 @@
+---
+module: [kind=guide] using_require
+---
+
+# Reusing code with require
+A library is a collection of useful functions and other definitions which is stored separately to your main program. You
+might want to create a library because you have some functions which are used in multiple programs, or just to split
+your program into multiple more modular files.
+
+Let's say we want to create a small library to make working with the @{term|terminal} a little easier. We'll provide two
+functions: `reset`, which clears the terminal and sets the cursor to (1, 1), and `write_center`, which prints some text
+in the middle of the screen.
+
+Start off by creating a file called `more_term.lua`:
+
+```lua {data-snippet=more_term}
+local function reset()
+  term.clear()
+  term.setCursorPos(1, 1)
+end
+
+local function write_center(text)
+  local x, y = term.getCursorPos()
+  local width, height = term.getSize()
+  term.setCursorPos(math.floor((width - #text) / 2) + 1, y)
+  term.write(text)
+end
+
+return { reset = reset, write_center = write_center }
+```
+
+Now, what's going on here? We define our two functions as one might expect, and then at the bottom return a table with
+the two functions. When we require this library, this table is what is returned. With that, we can then call the
+original functions. Now create a new file, with the following:
+
+```lua {data-mount=more_term:more_term.lua}
+local more_term = require("more_term")
+more_term.reset()
+more_term.write_center("Hello, world!")
+```
+
+When run, this'll clear the screen and print some text in the middle of the first line.
+
+## require in depth
+While the previous section is a good introduction to how @{require} operates, there are a couple of remaining points
+which are worth mentioning for more advanced usage.
+
+### Libraries can return anything
+In our above example, we return a table containing the functions we want to expose. However, it's worth pointing out
+that you can return ''anything'' from your library - a table, a function or even just a string! @{require} treats them
+all the same, and just returns whatever your library provides.
+
+### Module resolution and the package path
+In the above examples, we defined our library in a file, and @{require} read from it. While this is what you'll do most
+of the time, it is possible to make @{require} look elsewhere for your library, such as downloading from a website or
+loading from an in-memory library store.
+
+As a result, the *module name* you pass to @{require} doesn't correspond to a file path. One common mistake is to load
+code from a sub-directory using `require("folder/library")` or even `require("folder/library.lua")`, neither of which
+will do quite what you expect.
+
+When loading libraries (also referred to as *modules*) from files, @{require} searches along the *@{package.path|module
+path}*. By default, this looks something like:
+
+* `?.lua`
+* `?/init.lua`
+* `/rom/modules/main/?.lua`
+* etc...
+
+When you call `require("my_library")`, @{require} replaces the `?` in each element of the path with your module name, and
+checks if the file exists. In this case, we'd look for `my_library.lua`, `my_library/init.lua`,
+`/rom/modules/main/my_library.lua` and so on. Note that this works *relative to the current program*, so if your
+program is actually called `folder/program`, then we'll look for `folder/my_library.lua`, etc...
+
+One other caveat is loading libraries from sub-directories. For instance, say we have a file
+`my/fancy/library.lua`. This can be loaded by using `require("my.fancy.library")` - the '.'s are replaced with '/'
+before we start looking for the library.
+
+## External links
+There are several external resources which go into require in a little more detail:
+
+ - The [Lua Module tutorial](http://lua-users.org/wiki/ModulesTutorial) on the Lua wiki.
+ - [Lua's manual section on @{require}](https://www.lua.org/manual/5.1/manual.html#pdf-require).