WIP: Migrate Wiki content over to src/docs

docs/code_coverage.md

+# Categories of coverage
+
+  * <strong><font color='greeb'>executed</font></strong> - this line of code was hit during execution
+  * <strong><font color='orange'>instrumented</font></strong> - this line of code was part of the compilation unit, but not executed
+  * <strong><font color='red'>missing</font></strong> - in a source file, but not compiled.
+  * ignored - not an executable line, or a line we don't care about
+
+Coverage is calculated as `exe / (inst + miss)`. In general, lines that are in `miss` should be ignored, but our exclusion rules are not good enough.
+
+# Buildbots
+
+Buildbots are currently on the [experimental waterfall](http://build.chromium.org/buildbot/waterfall.fyi/waterfall). The coverage figures they calculate come from running some subset of the chromium testing suite.
+
+  * [Linux](http://build.chromium.org/buildbot/waterfall.fyi/builders/Linux%20Coverage%20(dbg)) - uses `gcov`
+  * [Windows](http://build.chromium.org/buildbot/waterfall.fyi/builders/Win%20Coverage%20%28dbg%29)
+  * [Mac](http://build.chromium.org/buildbot/waterfall.fyi/builders/Mac%20Coverage%20%28dbg%29)
+
+Also,
+  * [Coverage dashboard](http://build.chromium.org/buildbot/coverage/)
+  * [Example coverage summary](http://build.chromium.org/buildbot/coverage/linux-debug/49936/) - the coverage is calculated at directory and file level, and the directory structure is navigable via the **Subdirectories** table.
+
+# Calculating coverage locally
+
+TODO
+
+# Advanced Tips
+
+Sometimes a line of code should never be reached (e.g., `NOTREACHED()`). These can be marked in the source with `// COV_NF_LINE`. Note that this syntax is exact.