Clarify some docs.

1 files changed, 8 insertions, 0 deletions

diff --git a/doc/subprocess.rst b/doc/subprocess.rst
index 383c797..bcf5044 100644
--- a/doc/subprocess.rst
+++ b/doc/subprocess.rst
@@ -26,6 +26,14 @@ multiple coverage.py runs will each write their data to a distinct file.
 Configuring Python for subprocess coverage
 ------------------------------------------
 
+To measure coverage in subprocesses, you have to do two things: set a value for
+the ``COVERAGE_PROCESS_START`` environment variable, and then invoke
+:func:`coverage.process_startup`.  
+
+How you set ``COVERAGE_PROCESS_START`` depends on the details of how you create
+subprocesses.  As long as the environment variable is visible in your subprocess,
+it will work.
+
 You can configure your Python installation to invoke the ``process_startup``
 function in two ways: