Documentation: extensions top-level copy

Added very draft copy to the top-level "Extensions" section

documentation/extensions/index.md

@@ -2,9 +2,11 @@
 
 360Giving provides a core set of fields which are designed to make it useful and usable for people to collect, share, and analyse data about grantmaking.
 
-Publishers often collect and wish to publish information which is not part of the 360Giving standard and there are important topics that intersect with the concerns of grantmakers but which do not necessarily warrant an update to the standard by themselves. In these cases, it may be appropriate to develop and use an extension for 360Giving.
+Publishers sometimes wish to collect and publish information which is not part of the 360Giving standard. The standard permits publishers to use any number of "additional fields" in data to meet their needs, but these additional fields are not recognised by the standard. This means that they cannot be validated using tools such as the Data Quality Tool and different publishers including the same information may struggle to coordinate around how to name and structure the additional data.
 
-Extensions add fields and validation rules which allow 360Giving publishers to include additional data in their publication in a way that enables it to be checked for validity and quality, as well as integrate better into the 360Giving data ecosystem.
+Extensions address some of the issues with additional fields by introducing new optional fields and structures into 360Giving. Each extension formally defines fields, structures, and validation rules which are recognised by tooling such as the Data Quality Tool. This allows data using extensions to be validated, and also provides further standardisation which supports interoperability between different data sets.
+
+Extensions also address the issue of trying to support different use cases of 360Giving whilst maintaining the stability and integrity of the standard. A set of fields and features may be important to significant portion of the 360Giving community while there may not be enough demand to warrant adding the feature to the standard. Developing the features as an Extension allows for a meeting the needs of the community in a much shorter timescale, as well as test out how the features work in practice. It is possible that some Extensions may later be intergrated into the main standard.
 
 This section contains documentation about each extension for 360Giving.