docs: Add compiler flags in example (#22)

* Add '-pthread' and '$(pythia8-config --cxxflags)' to C++ example in the
  README to allow for compilation to proceed.
* Add missing '$USER' to 'id -g' to make it explicit which user info is used.

Author: matthewfeickert

README.md

@@ -33,7 +33,7 @@ You can either use the image as "`PYTHIA` as a service", as demoed here with the
 ```
 docker run \
   --rm \
-  --user $(id -u $USER):$(id -g) \
+  --user $(id -u $USER):$(id -g $USER) \
   --volume $PWD:/work \
   matthewfeickert/pythia-python:pythia8.308 \
   'python tests/main01.py > main01_out_py.txt'
@@ -44,10 +44,10 @@ or the original C++
 ```
 docker run \
   --rm \
-  --user $(id -u $USER):$(id -g) \
+  --user $(id -u $USER):$(id -g $USER) \
   --volume $PWD:/work \
   matthewfeickert/pythia-python:pythia8.308 \
-  'g++ tests/main01.cc -o tests/main01 $(pythia8-config --ldflags); ./tests/main01 > main01_out_cpp.txt'
+  'g++ tests/main01.cc -pthread -o tests/main01 $(pythia8-config --cxxflags --ldflags); ./tests/main01 > main01_out_cpp.txt'
 ```
 
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