travis: add doc

doc/travis-ci.txt

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+Support for using travis-ci has been added in order to automate pull-testing.
+See https://travis-ci.org/ for more info
+
+This procedure is different than the pull-tester that came before it in a few
+ways.
+
+There is nothing to administer. This is a major feature as it means
+that builds have no local state. Because there is no ability to login to the
+builders to install packages (tools, dependencies, etc), the entire build
+procedure must instead be controlled by a declarative script (.travis.yml).
+This script declares each build configuration, creates virtual machines as
+necessary, builds, then discards the virtual machines.
+
+A build matrix is constructed to test a wide range of configurations, rather
+than a single pass/fail. This helps to catch build failures and logic errors
+that present on platforms other than the ones the author has tested. This
+matrix is defined in the build script and can be changed at any time.
+
+All builders use the dependency-generator in the depends dir, rather than
+using apt-get to install build dependencies. This guarantees that the tester
+is using the same versions as Gitian, so the build results are nearly identical
+to what would be found in a final release. However, this also means that builds
+will fail if new dependencies are introduced without being added to the
+dependency generator.
+
+In order to avoid rebuilding all dependencies for each build, the binaries are
+cached and re-used when possible. Changes in the dependency-generator will
+trigger cache-invalidation and rebuilds as necessary.
+
+These caches can be manually removed if necessary. This is one of the the very few
+manual operations that is possible with Travis, and it can be done by the
+Bitcoin Core committer via the Travis web interface.
+
+In some cases, secure strings may be needed for hiding sensitive info such as
+private keys or URLs. The travis client may be used to create these strings:
+http://docs.travis-ci.com/user/encryption-keys/
+
+For the details of the build descriptor, see the official docs:
+http://docs.travis-ci.com/user/build-configuration/