Update readme for Aggregation feature and disabling parallelExecution

The current readme states

```
...it may be useful to disable parallel execution for the more resource intensive tasks like packageDaffodilBin or tests:

# run tests for root and all subprojects, disabling parallel execution of tests
# and creating saved parsers
sbt "set aggregate:= true" \
"set packageDaffodilBin / parallelExecution := false" \
"set test / parallelExecution := false" \
"test"
```

but this only sets it for the root, and each saved parser still runs in parallel, to get it to disable parallel execution, we need "every" as in

```
# run tests for root and all subprojects, disabling parallel execution of tests
# and creating saved parsers
sbt "set aggregate:= true" \
"set every packageDaffodilBin / parallelExecution := false" \
"set every test / parallelExecution := false" \
"test"
```

Open the repository README and find the documented sbt command for running tests and creating saved parsers with parallel execution disabled. Update both relevant settings to use the `every` scope as shown in the issue, then verify the README command matches the requested behavior and formatting.

Written by the indexing model from the issue text.