Add a bit of documentation

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/lcov.sh

@@ -1,5 +1,38 @@
 #!/bin/sh
 
+help () {
+    cat <<EOF
+Usage: $0
+Collect coverage statistics of library code into an HTML report.
+
+General instructions:
+1. Build the library with CFLAGS="--coverage -O0 -g3".
+   This can be an out-of-tree build.
+2. Run whatever tests you want.
+3. Run this script from the parent of the directory containing the library
+   object files and coverage statistics files.
+4. Browse the coverage report in Coverage/index.html.
+EOF
+}
+
+# Copyright The Mbed TLS Contributors
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -eu
+
+lcov_rebuild_stats () {
     rm -rf Coverage
     lcov --capture --initial --directory library -o files.info
     lcov --rc lcov_branch_coverage=1 --capture --directory library -o tests.info
@@ -8,3 +41,12 @@
     gendesc tests/Descriptions.txt -o descriptions
     genhtml --title "mbed TLS" --description-file descriptions --keep-descriptions --legend --branch-coverage -o Coverage final.info
     rm -f files.info tests.info all.info final.info descriptions
+    echo "Coverage report in: Coverage/index.html"
+}
+
+if [ $# -gt 0 ] && [ "$1" = "--help" ]; then
+    help
+    exit
+fi
+
+lcov_rebuild_stats