Small tweak and documentation to build wasmer-python

Author: brunokim

justfile

@@ -6,7 +6,7 @@ prelude:
 	pip3 install virtualenv
 	virtualenv .env
 	if test -d .env/bin/; then source .env/bin/activate; else source .env/Scripts/activate; fi
-	pip3 install maturin pytest pytest-benchmark twine pdoc
+	pip3 install maturin==0.12.20 pytest pytest-benchmark twine pdoc
 
 	which maturin
 	maturin --version

packages/api/README.md

@@ -266,6 +266,9 @@ $ python examples/appendices/simple.py
 
 ## Testing
 
+To build all tests you'll need LLVM 12.0 in your system. We recommend either installing prepackaged
+libraries with [llvm.sh](https://apt.llvm.org/llvm.sh) or building it yourself with [llvmenv](https://crates.io/crates/llvmenv).
+
 Build all the packages and run the tests:
 
 ```sh