Add content to guidelines.md

Make changes from #504 to live site.

Author: Amanda Butler

docs/reference/contributing/guidelines/guidelines.md

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 ## Guidelines
+
+Consistency is an important aspect of a codebase. To ensure consistency in Mbed OS code, we have created contributing guidelines. Any contribution to Mbed OS needs to meet the following criteria:
+
+#### Design and coding style
+
+Be consistent with your changes. We define software design principles and coding style in [this document](style.html).
+
+#### Contributions guidance
+
+The process how and how we review contributions is described in the [workflow](workflow.html) document.
+
+#### CLA
+
+All contributors must have signed [CLA](cla.html).
+
+#### Licenses
+
+License should comply with allowed [licenses described](license.html).