example-c.md

+The C example uses the `make` tool to build a simple C program with several compilers. As in the [Python example](python-example), we will make heavy use of the `ligo/software` image as a build environment.
+
+We'll make additional use of `ccache` to reuse `*.o` object files from previous builds when the C code has not changed. This particular example is so simple, it doesn't actually produce `.o` files, but ccache does significantly decrease build time for more complex software. You must include sections like this in your build to take advantage of `ccache`.
+```yaml
+before_script:
+  - export PATH=/usr/lib/ccache:/usr/lib64/ccache:$PATH
+  - export CCACHE_DIR=${CI_PROJECT_DIR}/ccache
+
+# each job's cache is identified by key and it helps to use a reasonably
+# unique key that might still be shared between forks of a project
+cache:
+  key: "$CI_PROJECT_NAME/$CI_JOB_NAME"
+  paths:
+    - ccache
+```
+The example jobs combine the building and testing into a single stage because each are so simple. If you have more complicated builds -- or want to test against more current releases of software dependencies -- it may make sense to split the pipeline into multiple stages [using build artifacts](using-build-artifacts).
+```yaml
+# test against GCC released in Debian 8 "jessie"
+jessie:
+  image: ligo/software:jessie
+  script:
+    - make V=1
+    - ./example
+
+# test against GCC released in RHEL7
+el7:
+  image: ligo/software:el7
+  script:
+    - make V=1
+    - ./example
+```
+We then test against a variety of [official DockerHub gcc images](https://hub.docker.com/_/gcc/) as well as with `clang`.
+```yaml
+# test against LLVM/clang released in Debian 8 "jessie"
+clang:
+  image: ligo/software:jessie
+  script:
+    - export CC="clang"
+    - make V=1
+    - ./example
+```
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