Updating the README with correct instructions

Hello there,

While cloning the project and following the instructions I'm getting this error:

```
|> mix do release, bootstrap, zip
warning: String.strip/1 is deprecated. Use String.trim/1 instead
/Users/h/workspace/aws-lambda-elixir-runtime/examples/hello_world/deps/poison/mix.exs:4

==> poison
Compiling 4 files (.ex)
warning: Integer.to_char_list/2 is deprecated. Use Integer.to_charlist/2 instead
lib/poison/encoder.ex:173

Generated poison app
==> aws_lambda_elixir_runtime
Compiling 15 files (.ex)
Generated aws_lambda_elixir_runtime app
==> artificery
Compiling 10 files (.ex)
Generated artificery app
==> distillery
Compiling 33 files (.ex)
Generated distillery app
==> hello_world
Compiling 1 file (.ex)
Generated hello_world app
** (Mix) The task "release" could not be found
```

I managed to compile something by using the distillery release command instead (and replacing the relevant parts), however, I feel that it's not the way to go, as I got this error while hitting the lambda:

```
START RequestId: e8fdcb89-d349-4d36-b392-fcf95e9cd250 Version: $LATEST
++ dirname /var/task/bootstrap
+ BASE=/var/task
+ EXE=/var/task/bin/hello_world
+ HOME=/tmp
+ export HOME
+ export RELEASE_READ_ONLY=true
+ RELEASE_READ_ONLY=true
+ export LD_LIBRARY_PATH=/lib64:/usr/lib64:/var/runtime:/var/runtime/lib:/var/task:/var/task/lib:/opt/lib:/var/task/lib/elixir_runtime-0.1.0/priv
+ LD_LIBRARY_PATH=/lib64:/usr/lib64:/var/runtime:/var/runtime/lib:/var/task:/var/task/lib:/opt/lib:/var/task/lib/elixir_runtime-0.1.0/priv
+ /var/task/bin/hello_world foreground
mkdir: cannot create directory ‘/var/task/var’: Read-only file system
END RequestId: e8fdcb89-d349-4d36-b392-fcf95e9cd250
REPORT RequestId: e8fdcb89-d349-4d36-b392-fcf95e9cd250 Duration: 231.23 ms Billed Duration: 300 ms Memory Size: 128 MB Max Memory Used: 13 MB
RequestId: e8fdcb89-d349-4d36-b392-fcf95e9cd250 Error: Runtime exited with error: exit status 1
Runtime.ExitError
```

Please, can we get some insights on this one?