tweak readme

README.md

@@ -1,9 +1,17 @@
-# Source Maps
+# Parcel's source-map library
 
 A purpose build source-maps library for combining and manipulating source-maps.
 
 For just reading source-maps, this library does not outperform the probably more stable and well-known package `source-map` by Mozilla.
 
+## Why did we write this library
+
+Parcel is a performance concious bundler, and therefore we like to optimise Parcel's performance as much as possible.
+
+Our original source-map implementation used mozilla's source-map and a bunch of javascript and had issues with memory usage and serialisation times (we were keeping all mappings in memory using JS objects and write/read it using JSON for caching).
+
+This implementation has been written from scratch in C++ minimizing the memory usage, by utilising indexes for sources and names and optimising serialisation times by using flatbuffers instead of JSON for caching.
+
 ## Compile flatbuffer schema
 
 ```bash