Added api.md for runes

- As asked @paly2 , here is the API (as accurate as possible)

mods/runes/api.md

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+RUNES' API
+==========
+
+(as accurate as possible)
+
+# 1. How it **does** work
+
+	Every rune registered uses the same method to work and access its environment. Runes are first registered using a specific function.
+Their datas are stored somewhere in a namespace (see below). You can use multiple values to indicate the needed amount of mana, the type of
+the rune, its texture(s), etc. Then, once a rune is registered (as an item, for minetest), the most important thing is to connect its handler(s).
+Another specific function is used to this, where you pass as an argument the reference to a function receiving the parameters of the item/node's callback
+(eg. on_punch, on_dig, on_use, etc). The handler can choose to return "true" in order to manage the use of mana by itself instead of letting the
+item's code in minetest.registered_items[<rune>].<callback> do it.
+
+# Functions
+
+## `runes.register_rune(parameters)`
+The most important function. This is the function you use to declare your rune and its specifications to minetest itself.
+It takes one argument, a table. This table can have the following articles :
+    - `name` is mandatory, it's the rune's name (which will be used to craft its itemstring : runes:rune_<name>_<level>)
+    - `desc` is the item's description
+    - `img` is another table :
+        - ["minor"] = <texture_file>
+	- ["medium"] = <texture_file>
+	- ["major"] = <texture_file>
+	None of these parameters are mandatory. Runes can have 1 or 2 or 3 levels (ok, not implemented yet, but it's coming).
+	You can use a single value like `img = <texture_file>` and the texture will be applied for all levels
+    - `type` is the type of rune. At the moment, three types are available :
+	- "craftitem" will register a craftitem
+	- "cube" will register a node
+	- "plate" will register a little slab 0.1 node thin
+    - `needed_mana` is yet another table with different levels :
+	- ["minor"] = <value>
+	- ["medium"] = <value>
+	- ["major"] = <value>
+	When using this rune at level "major", the ["major"] value of mana will be taken by default if the player has enough mana,
+	or it will be forbidden for him to use the rune if he doesn't have enough mana.
+	You don't have to indicate that parameter and just let the handler manage the mana.
+
+## `runes.functions.connect(itemname, callback, handler)`
+This function will connect a registered rune's callback to its handler. It takes three mandatory arguments :
+    - `itemname` is the rune's identification name (aka. `name` for earlier). The same handler is used for every level
+	of a rune, so it has to determine what level he will handle (it's actually passed to it)
+    - `callback` is a code identifying the callback. It can be :
+	- `use` for `on_use`
+	- `place` for `after_place_node`
+	- `dig` for `after_dig_node` (actually commented due to bugs in this section)
+	- `can_dig` for `can_dig` (true logic)
+	- `punch` for `on_punch` (rather logic too)
+    - `handler` is the function handling the callback. See below for details.
+Do not hesitate to consult lua_api.txt in order to see which callbacks are handled by what type of runes (craftitems, and nodes).
+
+## `runes.functions.register_amulet(name, desc, maxcount, manadiff)`
+Amulets are special items able to increase the maximum of mana you can stock. Here are the arguments expected to register an amulet :
+    - `name` is its name, used to create its itemstring : 'runes:<name>_amulet'
+	Don't feel forced to use '_amulet' in an amulet name, it's completly useless.    
+    - `desc` is the item description.
+    - `maxcount` represents the maximum amount of this amulet you can have per stack (aka `stack_max`, see lua_api.txt for this).
+    - `manadiff` is the amount of space you will get to stock more mana in your inventory. A global_step loop is charged with the task
+	of regulary looking into everyone's rune inventory to calculate every player's max_mana. If you loose amulets, your mana_max
+will dicrease (and mana may be lost with it).
+
+## `runes.functions.register_detached_inventory(player)`
+This function is only important for rune's inners. It registers a detached inventory to contain runes. Do not bother using it, there is no
+actual use of it out of runes' core.
+
+## `runes.functions.save_detached_inventory(player)`
+Saves player's rune inventory on hard drive (in minetest.get_worldpath() .. "/runes/" .. player:get_player_name() .. "_rune.inv").
+