[FIX] Reword front page (#958)

* [FIX] Reword front page.

* Use the word "standard" to describe what this is.

* Update src/index.md

Co-authored-by: Remi Gau <[email protected]>

* Update src/index.md

Co-authored-by: Remi Gau <[email protected]>

Co-authored-by: Remi Gau <[email protected]>
Co-authored-by: Stefan Appelhoff <[email protected]>

src/index.md

@@ -1,9 +1,17 @@
 # The Brain Imaging Data Structure
 
-This resource defines the Brain Imaging Data Structure (BIDS) specification, including the core specification as well as many modality-specific extensions.
+The Brain Imaging Data Structure (BIDS) is a simple and intuitive way to
+organize and describe data.
 
-To get started, [check out the introduction](01-introduction.md). If you'd like
-more information on how to adapt your own datasets to match the BIDS
-specification, we recommend exploring the [BIDS Starter Kit](https://bids-standard.github.io/bids-starter-kit/)
+This document defines the BIDS specification, which provides many details
+to help implement the standard. It includes the core specification as well as many extensions
+to specific brain imaging modalities, and increasingly also to other kinds of
+data.
 
-For an overview of the BIDS ecosystem, visit the [BIDS homepage](https://bids.neuroimaging.io).  The entire specification can also be [downloaded as PDF](https://doi.org/10.5281/zenodo.3686061).
+If BIDS is new to you, and you would like to learn more about how to adapt your
+own datasets to match the BIDS specification, we recommend exploring the
+[BIDS Starter Kit](https://bids-standard.github.io/bids-starter-kit/).
+Alternatively, to get started please read [the introduction to the specification](01-introduction.md).
+
+For an overview of the BIDS ecosystem, visit the [BIDS homepage](https://bids.neuroimaging.io).
+The entire specification can also be [downloaded as PDF](https://doi.org/10.5281/zenodo.3686061).