add the code of the join-existentials example

1 job for master
Status Job ID Name Coverage
  Test
failed #257
coq
buildjob

00:00:15

 
Name Stage Failure
failed
buildjob Test There is an unknown failure, please try again
/usr/bin/time -f "prelude/base (user: %U mem: %M ko)" "coqc"  -q  -Q "." iris   prelude/base
make[1]: *** No rule to make target 'examples/joining_existentials.v', needed by 'examples/joining_existentials.vo'. Stop.
make[1]: *** Waiting for unfinished jobs....
prelude/base (user: 0.87 mem: 369380 ko)
make[1]: Leaving directory '/builds/FP/iris-coq'
Makefile:3: recipe for target 'all' failed
make: *** [all] Error 2

ERROR: Build failed with: exit code 1