Document CI and OTP version management/pinning

doc/src/guide/book.asciidoc

@@ -30,6 +30,8 @@ include::sfx.asciidoc[Self-extracting releases]
 
 include::escripts.asciidoc[Escripts]
 
+include::kerl.asciidoc[OTP version management]
+
 include::compat.asciidoc[Compatibility with other build tools]
 
 [[docs]]

doc/src/guide/ci.asciidoc

@@ -1,6 +1,62 @@
 [[ci]]
 == Continuous integration
 
-// @todo Write it.
+Erlang.mk comes with some support for continuous integration,
+aimed at open source projects that need to support more than
+one specific Erlang/OTP release. (If you target one specific
+release, check the xref:otp_version_pinning[OTP version pinning]
+section of the xref:kerl[OTP version management] chapter.)
 
-Placeholder chapter.
+=== Configuring Erlang/OTP versions to test
+
+To use the CI plugin you must first configure which versions
+of Erlang/OTP will be used. Erlang.mk provides three separate
+configuration variables depending on whether you need a normal
+OTP release, a HiPE-enabled release or an ErLLVM-enabled release.
+
+At the time of writing, this is how you would test against all
+the most recent patch releases of Erlang/OTP 19 and above:
+
+[source,make]
+CI_OTP = OTP-19.0.7 OTP-19.1.6 OTP-19.2.3 OTP-19.3.6.2 OTP-20.0.4
+
+If you want to test against HiPE and ErLLVM but only with the
+latest version of Erlang/OTP, the following could work:
+
+[source,make]
+CI_HIPE = $(lastword $(CI_OTP))
+CI_ERLLVM = $(CI_HIPE)
+
+Consult the xref:kerl[OTP version management] chapter for more
+information about Erlang/OTP versions and customization of the
+Erlang/OTP builds.
+
+=== Running tests across all configured versions
+
+The recommended way to run the `ci` target is with the option
+`-k`. It will ensure that tests are run for all configured
+Erlang/OTP versions, even if there are errors:
+
+[source,bash]
+$ make ci -k
+
+=== Extending the CI targets
+
+The `ci` target can be extended. For example to run Dialyzer
+at the end of CI:
+
+[source,make]
+ci:: dialyze
+
+Additional setup can be done by extending the `ci-setup`
+target. This target is ran before testing each individual
+Erlang/OTP version.
+
+For example, to ensure dependencies are re-fetched/built
+before testing individual Erlang/OTP releases:
+
+[source,make]
+ci-setup:: distclean
+
+Similarly, the `ci-extra` target can be extended to run
+extra commands after an Erlang/OTP version has been tested.

doc/src/guide/kerl.asciidoc

@@ -0,0 +1,76 @@
+[[kerl]]
+== OTP version management
+
+Erlang.mk comes with integrated support for
+https://github.com/kerl/kerl[Kerl], a shell script that
+automates the downloading, building and installing of
+Erlang/OTP. It can be used to easily build a specific
+Erlang/OTP version (with or without custom build options)
+or maintain different versions side by side.
+
+=== Erlang versions
+
+Erlang.mk uses the Git tags from Erlang/OTP to identify
+OTP versions. The most recent tag at the time of writing
+is `OTP-20.0.4`, which is a patch release of `OTP-20.0`.
+A patch release is a non-official release containing a
+few fixes on top of the official release.
+
+Older versions used a slightly different versioning scheme
+and tag format, for example: `OTP_R16B03`. Beware though,
+there also was an `OTP_R16B03-1` release that fixed a
+critical issue in the initial release.
+
+The README file for all official Erlang/OTP releases can
+be found on http://www.erlang.org/downloads[erlang.org].
+To obtain information about patch releases when they are
+released you need to be subscribed to the
+http://erlang.org/mailman/listinfo/erlang-questions[erlang-questions mailing list].
+
+[[otp_version_pinning]]
+=== OTP version pinning
+
+Erlang.mk can use a specific version of Erlang/OTP when
+interacting with your project. This can be very useful
+when you are working with a team because you can define
+the version you need in the Makefile and Erlang.mk will
+ensure this version is used by everyone in your team.
+
+To pin the version all you need to do is to set the
+`ERLANG_OTP` variable in your Makefile before including
+Erlang.mk. For example, to use `OTP-20.0.4`:
+
+[source,make]
+----
+ERLANG_OTP = OTP-20.0.4
+
+include erlang.mk
+----
+
+The next time you run `make` Erlang.mk will build and
+use the version you configured.
+
+Note that there has been reports that this functionality
+is not compatible with the `.ONESHELL` feature from
+GNU Make.
+
+=== Continuous integration
+
+Erlang.mk can automatically test your project against
+many different Erlang/OTP versions. This functionality
+is documented in the xref:ci[Continuous integration] chapter.
+
+=== Configuring Kerl
+
+All of the Kerl variables can be configured directly in the
+Makefile. All you need to do is to export them. For example,
+to change the installation directory for the Erlang/OTP
+versions managed by Kerl, you could add the following to
+your Makefile:
+
+[source,make]
+export KERL_INSTALL_DIR = $(CURDIR)/erlang
+
+When configuring paths like this, always make sure to provide
+an absolute path in the value. Erlang.mk will NOT expand them
+automatically for you.