add README explaining purpose of include/ directory

(cherry picked from commit f85fc457)
31 jobs for prep-lalsimulation-2.1.0 in 127 minutes and 8 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  .Pre
passed #830735
make-dist

00:06:09

 
  Lal
passed #830740
lal:conda:fftw

00:12:01

passed #830742
lal:conda:mkl

00:13:32

passed #830739
lal:deb

00:28:58

passed #830737
lal:rpm

00:19:50

 
  Lal Frame
passed #830748
lalframe:conda

00:10:19

passed #830746
lalframe:deb

00:09:29

failed #830744
lalframe:rpm

00:07:45

 
  Lal Meta Io
passed #830753
lalmetaio:conda

00:08:06

passed #830752
lalmetaio:deb

00:09:31

passed #830750
lalmetaio:rpm

00:07:29

 
  Lal Simulation
passed #830758
lalsimulation:conda

00:12:01

passed #830757
lalsimulation:deb

00:22:30

passed #830755
lalsimulation:rpm

00:13:08

 
  Lal Burst
passed #830764
lalburst:conda

00:12:08

passed #830762
lalburst:deb

00:17:32

passed #830760
lalburst:rpm

00:20:02

 
  Lal Inspiral
passed #830770
lalinspiral:conda

00:14:22

passed #830768
lalinspiral:deb

00:16:53

skipped #830766
lalinspiral:rpm
 
  Lal Pulsar
passed #830775
lalpulsar:conda

00:13:31

passed #830774
lalpulsar:deb

00:18:31

passed #830772
lalpulsar:rpm

00:14:48

 
  Lal Inference
passed #830780
lalinference:conda

00:19:00

passed #830779
lalinference:deb

00:18:56

skipped #830777
lalinference:rpm
 
  Lal Apps
passed #830787
lalapps:conda

00:16:05

passed #830786
lalapps:deb

00:15:59

skipped #830784
lalapps:rpm
 
  Documentation
passed #830788
documentation

00:05:51

 
  Integration Tests
passed #830789
top-level

00:43:01

 
Name Stage Failure
failed
lalframe:rpm Lal Frame
WARNING: rpmbuild/BUILD/*/test/test-suite.log: no matching files 
rpmbuild/BUILD/*/test/python/test-suite.log: found 1 matching files

WARNING: rpmbuild/RPMS/x86_64/lalframe-*.rpm: no matching files
WARNING: rpmbuild/RPMS/x86_64/python*-lalframe-*.rpm: no matching files
Uploading artifacts to coordinator... ok
id=830744 responseStatus=201 Created token=AVJxBPyH
Uploading artifacts...
WARNING: rpmbuild/BUILD/*/test/python/junit*.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1