Merge pull request #9 from nsajko/readme_type_stability

readme: add some clarifications regarding return types

Author: MasonProtter

README.md

@@ -29,6 +29,14 @@ Stacktrace:
    @ REPL[3]:1
 ```
 
+Note regarding the return type of `@spawn`:
+* `Threads.@spawn` returns a value of the concrete type `Task`
+* In contrast, the return type of `StableTasks.@spawn` may depend on any and all of the following:
+    * the configuration of type inference in the Julia compiler
+    * the return type of the input function
+        * in particular, assuming type inference is precise, `typeof(StableTasks.@spawn 3) != typeof(StableTasks.@spawn 3.0)`, because `typeof(3) !== typeof(3.0)`
+* You should **not** write code that relies upon the exact return type of `StableTasks.@spawn` being concrete, or stable across versions. The return type in its parameter is available **only** as an optimization.
+
 ## `StableTasks.@spawnat`
 
 The package also provides `StableTasks.@spawnat`, which is similar to `StableTasks.@spawn` but creates a *sticky* task (that won't migrate) on a specific thread.
@@ -40,6 +48,8 @@ julia> @inferred fetch(t)
 4
 ```
 
+The note regarding the return type of `@spawn` also applies here.
+
 ## `StableTasks.@fetch` and `StableTasks.@fetchfrom`
 
 For convenience, and in analogy to at Distributed.jl, we also provide `@fetch` and `@fetchfrom` macros: