Merge pull request getodk#283 from lognaturel/master

Add section on code from external sources to the contribution guide.

CONTRIBUTING.md

@@ -31,4 +31,9 @@ To contribute code to ODK Collect, you will need to open a [pull request](https:
 For now, match the style of the code in the file you are editing. When creating new files, follow the [Android style rules](http://source.android.com/source/code-style.html).
 
 ## Strings
-Always use [string resources](https://developer.android.com/guide/topics/resources/string-resource.html) instead of literal strings. This ensures wording consistency across the project and also enables full translation of the app. Only make changes to the base `res/values/strings.xml` English file and not to the other language files. The translated files are generated from [Transifex](https://www.transifex.com/opendatakit/odk-collect/) where translations can be submitted by the community. Names of software packages or other untranslatable strings should be placed in `res/values/untranslated.xml`.
+Always use [string resources](https://developer.android.com/guide/topics/resources/string-resource.html) instead of literal strings. This ensures wording consistency across the project and also enables full translation of the app. Only make changes to the base `res/values/strings.xml` English file and not to the other language files. The translated files are generated from [Transifex](https://www.transifex.com/opendatakit/odk-collect/) where translations can be submitted by the community. Names of software packages or other untranslatable strings should be placed in `res/values/untranslated.xml`.
+
+## Code from external sources
+ODK Collect is released under the [Apache 2.0](https://www.apache.org/licenses/LICENSE-2.0) license. Please make sure that any code you include is licensed in a compatible way. **Please note that if no license is specified for a piece of code or if it has an incompatible license such as GPL, using it puts the project at legal risk**.
+
+Sites with compatible licenses (including [StackOverflow](http://stackoverflow.com/)) will sometimes provide exactly the code snippet needed to solve a problem. You are encouraged to use such snippets in ODK Collect as long as you attribute them by including a direct link to the source. In addition to complying with the content license, this provides useful context for anyone reading the code.