bug 1532344: geckodriver: document how to self-service arm7hf builds; r=me a=doc

DONTBUILD

testing/geckodriver/doc/ARM.md

@@ -0,0 +1,39 @@
+Self-serving an ARM build
+=========================
+
+Mozilla [announced the intent] to deprecate ARMv7 HF builds of
+geckodriver in September 2018.  This does not mean you can no longer
+use geckodriver on ARM systems, and this document explains how you
+can self-service a build for ARMv7 HF.
+
+Assuming you have already checked out [central], the steps to
+cross-compile ARMv7 from a Linux host system is as follows:
+
+  1. If you don’t have Rust installed:
+
+         # curl https://sh.rustup.rs -sSf | sh
+
+  2. Install cross-compiler toolchain:
+
+         # apt install gcc-arm-linux-gnueabihf libc6-armhf-cross libc6-dev-armhf-cross
+
+  3. Createa a new shell, or to reuse the existing shell:
+
+         source $HOME/.cargo/env
+
+  4. Install rustc target toolchain:
+
+         % rustup target install armv7-unknown-linux-gnueabihf
+
+  5. Put this in testing/geckodriver/.cargo/config:
+
+         [target.armv7-unknown-linux-gnueabihf]
+         linker = "arm-linux-gnueabihf-gcc"
+
+  6. Build geckodriver from testing/geckodriver:
+
+         % cd testing/geckodriver
+         % cargo build --release --target armv7-unknown-linux-gnueabihf
+
+[announce the intent]: https://lists.mozilla.org/pipermail/tools-marionette/2018-September/000035.html
+[central]: https://hg.mozilla.org/mozilla-central/

testing/geckodriver/doc/index.rst

@@ -42,6 +42,7 @@ For developers
    Building.md
    Testing.md
    Releasing.md
+   ARM.md
 
 
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