Update README.md (#255)

Co-authored-by: Rachel M. Carmena <[email protected]>

Author: 1Jajen1

arrow-libs/core/arrow-docs/docs/quickstart/libraries/README.md

@@ -19,6 +19,7 @@ In this doc, we'll describe all the modules that form the core, alongside a list
 * [arrow-core-data]({{ '/quickstart/libraries/#arrow-core-data' | relative_url }})
 * [arrow-optics]({{ '/quickstart/libraries/#arrow-optics' | relative_url }})
 * [arrow-fx-coroutines]({{ '/quickstart/libraries/#arrow-fx-coroutines' | relative_url }})
+* [arrow-fx-stm]({{ '/quickstart/libraries/#arrow-fx-stm' | relative_url }})
 * [arrow-syntax]({{ '/quickstart/libraries/#arrow-syntax' | relative_url }})
 * [arrow-fx-rx2 & arrow-fx-reactor]({{ '/quickstart/libraries/#arrow-fx-rx2&arrow-fx-reactor' | relative_url }})
 * [arrow-mtl]({{ '/quickstart/libraries/#arrow-mtl' | relative_url }})
@@ -80,11 +81,25 @@ dependencies {
 }
 ```
 
-The [fx library]({{ '/fx/' | relative_url }}) is a Kotlin idiomatic functional effects stytem that offers a powerful set concurrency combinators and primitives with an emphasis on easy concurrency and parallelism with guarantees about concurrent and parallel resource safety and composition.
+The [fx library]({{ '/fx/' | relative_url }}) is a Kotlin idiomatic functional effects system that offers a powerful set concurrency combinators and primitives with an emphasis on easy concurrency and parallelism with guarantees about concurrent and parallel resource safety and composition.
 If offers a set of integrations for `KotlinX Coroutines`, `RxJava`, `Reactor`, etc. Check the [Arrow Fx documentation](({{ '/fx/' | relative_url }})) for more details.
 
 Dependency: `arrow-core`
 
+### arrow-fx-stm
+
+```groovy
+dependencies {
+    implementation "io.arrow-kt:arrow-fx-stm:$arrow_version"
+}
+```
+
+The *STM* module provides powerful combinators to build safe stateful transactions in a concurrent settings. Transactions are guaranteed to not cause deadlocks or race conditions. This guarantee also carries over on composition which enables users to build everything from single transactions to fully concurrent datastructures.
+
+The module also provides several common datastructures such as Queues, Semaphores, Maps and more.
+
+Dependency: `arrow-fx-coroutines`
+
 ## Extension libraries
 
 These libraries are hosted inside the Arrow repository building on the core, to provide higher level constructs to deal with concepts rather than code abstraction.