doc: add a note about required lcov version

Add a note about min. version of lcov required with intermediate text
format support.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

doc/guides/coverage.rst

@@ -122,6 +122,13 @@ You may postprocess these with your preferred tools. For example:
    lcov --capture --directory ./ --output-file lcov.info -q --rc lcov_branch_coverage=1
    genhtml lcov.info --output-directory lcov_html -q --ignore-errors source --branch-coverage --highlight --legend
 
+.. note::
+
+   You need a recent version of lcov (at least 1.14) with support for
+   intermediate text format. Such packages exist in recent Linux distributions.
+
+   Alternatively, you can use gcovr (at least version 4.2).
+
 Sanitycheck coverage reports
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