Add basic metaschema tutorial for usnistgov#260.

website/content/tutorials/_index.md

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-Metaschema is a framework for modeling information into data formats. To start, let us imagine we want to exchange information about computers. How do we model a computer in Metaschema? We start with a metaschema definition.
+Metaschema is a framework for modeling information into machine-readable data formats. If we want to build tools to exchange information about computers, how do we model a computer in Metaschema and what are the benefits?
+
+We start with an empty metaschema definition, like the one below.
 
 ```xml
-<METASCHEMA></METASCHEMA>
+<?xml version="1.0" encoding="UTF-8"?
+<METASCHEMA xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0">
+</METASCHEMA>
+```
+
+We write a Metaschema definition in XML. An empty definition like above has a beginning and ending `METASCHEMA` tags. We want to add useful metadata for both developers and Metaschema-enabled tools to consume this definition, like so.
+
+```xml
+<METASCHEMA xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0">
+  <schema-name>Computer Model</schema-name>
+  <schema-version>0.0.1-alpha</schema-version>
+  <short-name>computer</short-name>
+  <namespace>http://example.com/ns/computer</namespace>
+</METASCHEMA>
+```
+
+The metadata above provides useful information to the developer and tools. The `schema-name` is the long-form, descriptive name of the computer model. The `schema-version` is to give the model itself a version number, for either developers or their tools to use. The `short-name` is the shortened form of the `schema-name`, and most importantly the outputs of Metaschema-enabled tools will look for data fields with the name `computer`, not `Computer Model`. The `namespace` is a URI used to identify the model and its parts as belonging to a single scope, like a module name or class name in other data formats and programming languages.
+
+With a minimal metadata section in place, we will add the first part of the model for the computer, an `assembly` for a computer itself.
+
+```xml
+<METASCHEMA xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0">
+  <schema-name>Computer Model</schema-name>
+  <schema-version>0.0.1-alpha</schema-version>
+  <short-name>computer</short-name>
+  <namespace>http://example.com/ns/computer</namespace>
+
+  <define-assembly></define-assembly>
+</METASCHEMA>
 ```