Add psr-15 example to examples folder

[shish]

Add psr-15 example to examples folder

This example is broken and I don't know why (maybe somebody who knows what they are doing can explain?). But at least now we can see that it is broken, which is better than just silently misleading people...

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.31%. Comparing base (53f9d49) to head (a705281).
Report is 84 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #669      +/-   ##
============================================
- Coverage     95.72%   95.31%   -0.41%     
- Complexity     1773     1809      +36     
============================================
  Files           154      171      +17     
  Lines          4586     4783     +197     
============================================
+ Hits           4390     4559     +169     
- Misses          196      224      +28     

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[shish]

So this is failing and I don't know why :( I tried the same fix which worked for the no-framework example, but no luck

$ cd examples/psr-15
$ composer install
$ php -S 127.0.0.1:8080

$ curl -X POST -d '{"query":"{ hello(name: \"World\") }"}' -H "Content-Type: application/json" http://localhost:8080/graphql
{"errors":[{"message":"Cannot query field \"hello\" on type \"Query\".","locations":[{"line":1,"column":3}]}]}%