add a docs page on build filters (#3233)

This is mostly a straight copy from the changelog, but more discoverable.

docs/partial_builds.md

@@ -0,0 +1,81 @@
+# Build Filters
+
+Build filters allow you to choose explicitly which files to build instead of
+building entire directories or projects.
+
+A build filter is a combination of a package and a path, with glob syntax
+supported for each.
+
+Whenever a build filter is provided, only required outputs matching one of the
+build filters will be built, in addition to any required files for those outputs.
+
+## Command Line Usage
+
+Build filters are supplied using the `--build-filter` option, which accepts
+relative paths under the root package as well as `package:` uris.
+
+Glob syntax is allowed in both package names and paths.
+
+**Example**: The following would build and serve the JS output for an
+application, as well as copy over the required SDK resources for that app:
+
+```
+pub run build_runner serve \
+  --build-filter="web/main.dart.js" \
+  --build-filter="package:build_web_compilers/**/*.js"
+```
+
+## Common Use Cases
+
+**Note**: For all the listed use cases it is up to the user or tool the user is
+using to request all the required files for the desired task. This package only
+provides the core building blocks for these use cases.
+
+### Testing
+
+If you have a large number of tests but only want to run a single one you can
+now build just that test instead of all tests under the `test` directory.
+
+This can greatly speed up iteration times in packages with lots of tests.
+
+**Example**: This will build a single web test and run it:
+
+```
+pub run build_runner test \
+  --build-filter="test/my_test.dart.browser_test.dart.js" \
+  --build-filter="package:build_web_compilers/**/*.js" \
+  -- -p chrome test/my_test.dart
+```
+
+**Note**: If your test requires any other generated files (css, etc) you will
+need to add additional filters.
+
+### Applications
+
+This feature works as expected with the `--output <dir>` and the `serve`
+command.  This means you can create an output directory for a single
+application in your package instead of all applications under the same
+directory.
+
+The serve command also uses the build filters to restrict what files are
+available, which means it ensures if something works in serve mode it will
+also work when you create an output directory.
+
+**Example**: This will build a single web app and serve it:
+
+```
+pub run build_runner serve \
+  --build-filter="web/my_app.dart.js" \
+  --build-filter="package:build_web_compilers/**/*.js"
+```
+
+**Note**: Your app may rely on additional code generated files (such as css
+files), which you will need to list as additional filters.
+
+### Build Daemon Usage
+
+The build daemon accepts build filters when registering a build target. If no
+filters are supplied these default filters are used:
+
+- `<target-dir>/**`
+- `package:*/**`