Include instructions to run SystemInfoTest

[ckadner]

It is not very straight forward to run the advertised SystemInfoTest from the command line unless one is eager to download a few jars and add them to the classpath (SLF4J, JNI, JSON).

For people familiar with Git and Maven, running a mvn command to execute SystemInfoTest will require less effort ...and if included in the README it comes down to a simple copy and paste 😄

mvn test-compile -pl oshi-core -q exec:java \
  -Dexec.mainClass="oshi.SystemInfoTest" \
  -Dexec.classpathScope="test"

[codecov-io]

Current coverage is 84.62% (diff: 100%)

Merging #301 into master will not change coverage

@@             master       #301   diff @@
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  Files            23         23          
  Lines          1080       1080          
  Methods           0          0          
  Messages          0          0          
  Branches        195        195          
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  Hits            914        914          
  Misses           72         72          
  Partials         94         94          

Powered by Codecov. Last update 4e654e6...1ac11a3

[coveralls]

Coverage remained the same at 92.222% when pulling 1ac11a3 on ckadner:how_to_run_SystemInfoTest_with_mvn into 4e654e6 on oshi:master.