Updating the README with correct instructions

Start with the README instructions around mix do release, bootstrap, zip and compare them with the Distillery release command mentioned in the report. Then inspect bootstrap and the Lambda log, especially the attempt to write under /var/task; done means the README gives a verified build and deployment path that avoids the reported failures.

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?