Add documentation for automatic builds

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

README.md

@@ -49,3 +49,4 @@ Miscellaneous
 - [Testing and Code Coverage](doc/make/coverage.md)
 - Building Doxygen Documentation: `doxygen src/Doxyfile`
 - [Coding style](doc/style.md)
+- [Automatic builds](doc/make/travis.md)

doc/make/travis.md

@@ -0,0 +1,15 @@
+Automatic Builds at [Travis](https://travis-ci.org/)
+----------------------------------------------------
+
+Whenever a commit is submitted to the the main repository,
+a service hook at github will automatically build Lean using
+[Travis](https://travis-ci.org/).
+
+Automatic builds can be enabled for any *fork* of the Lean repository.
+We just have to go to our fork page at github; select `settings` (it is an icon on the right hand side of the page);
+select `Service Hooks`; select `Travis`; and provide the required information.
+We must have an account at Travis.
+
+By default, build reports are sent to the [leansmt-dev](https://groups.google.com/forum/#!forum/leansmt-dev) Google group.
+You can change that by modifying the file [.travis.yml](../../.travis.yml) in the Lean root directory. The file contains a
+`recipients:` entry.