update engine readme.
## engine
-Engine is the collection of low-level crates with which application architects can build higher-level implementations of Strongholds for a variety of purposes. It is platform agnostic, in that it should run anywhere a Rust Compiler will work.
+Engine is the collection of low-level modules with which application architects can build higher-level implementations of Strongholds for a variety of purposes. It is platform agnostic, in that it should run anywhere a Rust Compiler will work.
-It is composed of 4 primary crates:
+It is composed of 4 primary modules:
- snapshot
- vault
- store
- runtime
+
+For more information on each of these modules, see their associated READMEs.
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