Add a section on interoperability

Author: chouzar

Diff for: Interoperability.md

@@ -0,0 +1,26 @@
+# Gleam's Interoperability with the BEAM
+
+## Effortlesly download dependencies from Erlang or Elixir
+
+We can take advantage of a whole ecosystem of libraries.
+
+```toml
+cubdb = ">= 2.0.0 and < 3.0.0"
+ecto = ">= 3.0.0 < 4.0.0"
+phoenix = ">= 1.7.0 and < 2.0.0"
+```
+
+## Compile both erlang or elixir from the Gleam tool
+
+Gleam wil compile both your erlang or elixir files.
+
+```sh
+gleam build
+```
+
+## Simple FFI to reach any function on the BEAM
+
+```gleam
+@external(erlang, "observer", "start")
+fn observer() -> x
+```