add README file and documentation

README.md

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+# Basic Materials mod
+
+mod that adds basic material nodes and items
+
+## Install
+
+### Option 1: use content tab in minetest
+
+* click the content tab
+* search for basic materials
+* click install on basic materials
+
+### Option 2: download from [ContentDB](https://content.minetest.net/packages/VanessaE/basic_materials/)
+
+* click the above link or search for basic materials on ContentDB
+* select the download button on the basic materials page
+* extract the zip downloaded from the previous step
+* place in your mods folder
+
+### Option 3: use git
+
+* `git clone https://github.com/mt-mods/basic_materials.git`
+* `cd basic_materials`
+* `git submodule init`
+* `git submodule update`
+
+to update please use the following commands starting inside the mod directory
+
+* `git submodule sync`
+* `git submodule update`