add sonar-scanner-cli docs (#8)

sonar-scanner-cli/content.md

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+# What is SonarScanner CLI?
+
+[SonarScanner](https://redirect.sonarsource.com/doc/install-configure-scanner.html) is the official scanner used to run code analysis on SonarQube and SonarCloud.
+
+# How to use this image
+
+To analyze your repository, you have to:
+
+-	Define your configuration in the `sonar-project.properties` file in the root of your source code repository.
+-	Pass your source code to the scanner. Your source code repository should be mounted to the container `/usr/src` directory.
+-	Pass your SonarQube or SonarCloud URL in the SONAR_HOST_URL environment variable. The default URL is `http://localhost:9000`, but you'll most likely need to change it to fit your setup.
+-	Run the following command:
+
+```console
+docker run \
+    --rm \
+    -e SONAR_HOST_URL=http://${SONARQUBE_URL} \
+    -v "${YOUR_REPO}:/usr/src" \
+    sonar-scanner-cli
+```
+
+For more details on how to use the image, visit the [SonarScanner](https://redirect.sonarsource.com/doc/install-configure-scanner.html) page in the SonarQube documentation.

sonar-scanner-cli/get-help.md

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+[the SonarSource Community forum](https://community.sonarsource.com/tags/c/help/sq/docker), [the Docker Community Forums](https://forums.docker.com/), [the Docker Community Slack](https://blog.docker.com/2016/11/introducing-docker-community-directory-docker-community-slack/), or [Stack Overflow](https://stackoverflow.com/search?tab=newest&q=docker)

sonar-scanner-cli/github-repo

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+https://github.com/SonarSource/sonar-scanner-cli-docker

sonar-scanner-cli/license.md

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+View [license information](http://www.gnu.org/licenses/lgpl.txt) for the software contained in this image.

sonar-scanner-cli/maintainer.md

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+[SonarSource](%%GITHUB-REPO%%)