Add warning about reticulate. (#863)

Add warning about R package reticulate.

The warning is a message (printed through R's `cat()`)
when the R package is loaded.

rpy2/robjects/robject.py

@@ -10,6 +10,30 @@
 rpy2.rinterface.initr_simple()
 
 
+def _add_warn_reticulate_hook():
+    msg = """
+    WARNING: While the R package "reticulate" was developped after
+    Python-R bridges such as rpy2 already existed, it does not
+    consider that it could be called from a Python process. This
+    results in a quasi-obligatory segfault when rpy2 is evaluating
+    R code using reticulate. On the hand, rpy2 is accounting for the
+    fact that it might already be running embedded in a Python
+    process. This is why:
+    - Python -> rpy2 -> R -> reticulate: crashes
+    - R -> reticulate -> Python -> rpy2: works
+
+    The issue with reticulate is tracked here:
+    https://github.com/rstudio/reticulate/issues/208
+    """
+    rpy2.rinterface.evalr(f"""
+    setHook(packageEvent("reticulate", "onLoad"),
+            function(...) cat({repr(msg)}))
+    """)
+
+
+_add_warn_reticulate_hook()
+
+
 class RSlots(object):
     """ Attributes of an R object as a Python mapping.