Travis-CI: targets update, compilation matrix with sizes, documentation

docs/travis-ci.md

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+# Travis-CI and Optiboot
+
+## What is Travis?
+
+https://travis-ci.com is site which allows to make checking builds after
+every push of code to git repository. It could also check pull request.
+
+It nicely integrates with Github, so to enable it's enough to just
+install Travis' Github App from: https://github.com/marketplace/travis-ci
+
+Travis have also free plan for Open Source projects, so there is no
+cost involved.
+
+When installed, after every push a build is started on Travis-CI
+servers. After build finish, status of commit is updated, so everyone
+can see if build passed or failed. Clicking on details gives more
+information about which job(s) within all build failed and even direct
+output from virtual machine, so it's easy to check why something went
+wrong.
+
+Of course, Travis-CI also needs configuration to define what should
+be run to test code, so every project must have *.travis.yml* file
+and some others if external scripts are needed.
+
+## Files
+
+### .travis.yml
+
+Standard Travis-CI file with definitions of jobs.
+
+There are 2 stages of tests:
+- *check sizes*: there is only one build in this stage which compiles ALL
+targets specified in *.travis.yml* (variable *OPTIBOOT_TARGET* in matrix)
+agains ALL compilers marked with 'yes' from *arduino-gcc-versions.md* file
+plus latest Microchip's compiler for avr8. There are 2 tables produced
+at the end of build: one is output of current compilation and resulting
+sizes for every target or 'x' if build failed. Second one is based on
+online database of builds and gives comparison between current build
+and last commited into repository. Output is markdown compatible, so
+it could be just copied and pasted into Github comment to produce
+pretty table.
+- test: stage with separate build for every *OPTIBOOT_TARGET* against only
+one compiler specified in variable *TOOLS_VERSION*. In 'allow_failures'
+section there are targets known to fail and ignored in overall status
+of test.
+
+### scripts/travis-build.sh
+
+Script used to build single target during 'test' stage.
+
+## scripts/travis-check-sizes.sh
+
+Script used to build all targets using all compilers during 'check sizes'
+stage.
+
+## scripts/travis-download.inc.sh
+
+Include script used to download and unpack tools. Used by *travis-build.sh*
+and *travis-check-sizes.sh* scripts.
+
+## scripts/travis-env.inc.sh
+
+Helper script for local testing (on Linux) builds. It sets all necessary
+environment variables to make *travis-build.sh* or *travis-check-sizes.sh*
+on local machine. Need to be sourced from repository parent directory:
+```bash
+. scripts/travis-env.inc.sh
+```
+
+## scripts/travis-fill-cache.sh
+
+Helper script used by *travis-check-sizes.sh* to download and extract all
+needed compilers. It uses *docs/arduino-gcc-versions.md* file to find
+which Arduino version should be downloaded.