Add instructions on how to format code

[skip ci]

.gitlab-ci.yml

@@ -4,7 +4,7 @@ check_formatting:
   stage: check
   image: gitlab-registry.cern.ch/scouting-demonstrator/scouting-docker-images/scouting-devtools:1.0.0
   script:
-    - find src/ -iname '*.h' -o -iname '*.cc' | xargs clang-format --style=file --dry-run -Werror
+    - find src/ -iname '*.h' -o -iname '*.cc' | xargs clang-format --style=file --dry-run -Werror || printf "Please reformat your code using the command\ngit clang-format\n\nafter staging your files with \`git add\` or\n\nclang-format -i [your files]\n"
 
 build_rpm_tag:
   stage: build

README.md

 # SCDAQ
 Rudimentary data taking application for the CMS L1 scouting demonstrator
 
-An RPM is built automatically by the CI pipeline and is stored as artifact of the build stage. **Important:** Once installed, the SCDAQ config file needs to be modified as described in the section below.
+An RPM is built automatically by the CI pipeline and is stored as artifact of the "build" stage. **Important:** Once installed, the SCDAQ config file needs to be modified as described in the section below.
 
 The RPM installs a systemd service that can be started with
 
@@ -15,6 +15,16 @@ if SCDAQ should be automatically started at boot it can be enabled with
 $ sudo systemctl enable runSCdaq
 ```
 
+## Developing
+
+We try to adhere to the [Google C++ style guide](https://google.github.io/styleguide/cppguide.html).
+
+### Code formatting
+
+The C++ code needs to follow the directives set forth in the `.clang-format` file placed in the repository root. This is enforced by the "check" stage of the CI.
+
+You can format your code either with `clang-format -i [modified file]`, or (in particular for multiple changed files) with `git clang-format` after you have staged the modified files.
+
 ## Manual operation
 
 ### Build manually