Add README describing use of semantic convention YAML models (open-te…

…lemetry#980)

semantic_conventions/README.md

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+# YAML Model for Semantic Conventions
+
+The YAML descriptions of semantic convention contained in this directory are intended to
+be used by the various OpenTelemetry language implementations to aid in automatic
+generation of semantics-related code.
+
+## Generation
+
+These YAML files are used by the make target `table-generation` to generate consistently
+formattted Markdown tables for all semantic conventions in the specification. Run it from the root of this repository using the command
+
+```
+make table-generation
+```
+
+For more information, see the [semantic convention generator](https://github.com/open-telemetry/build-tools/tree/master/semantic-conventions)
+in the OpenTelemetry build tools repository.
+Using this build tool, it is also possible to generate code for use in OpenTelemetry
+language projects.
+
+See also:
+
+* [Markdown Tables](https://github.com/open-telemetry/build-tools/tree/master/semantic-conventions#markdown-tables)
+* [Code Generator](https://github.com/open-telemetry/build-tools/tree/master/semantic-conventions#code-generator)
+
+## Description of the model
+
+The fields and their expected values are presented in [syntax.md](./syntax.md).