Add to documentation list of supported platforms and their tripl… (#622)

Author: giordano

docs/src/FAQ.md

@@ -24,6 +24,15 @@ Absolutely!  There's nothing Julia-specific about the binaries generated by the
 
 At the time of writing, we support Linux (`x86_64`, `i686`, `armv7l`, `aarch64`, `ppc64le`), Windows (`x86_64`, `i686`), macOS (`x86_64`) and FreeBSD (`x86_64`).
 
+You can get the list of the supported platforms and their associated _triplets_ by using the functions `supported_platforms` and `triplet`:
+
+```@repl
+using BinaryBuilder
+for p in supported_platforms()
+    println("Platform: $(p); triplet: $(triplet(p))")
+end
+```
+
 ### At line XXX, ABORTED (Operation not permitted)!
 
 Some linux distributions have a bug in their `overlayfs` implementation that prevents us from mounting overlay filesystems within user namespaces.  See [this Ubuntu kernel bug report](https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1531747) for a description of the situation and how Ubuntu has patched it in their kernels.  To work around this, you can launch `BinaryBuilder.jl` in "privileged container" mode.  BinaryBuilder should auto-detect this situation, however if the autodetection is not working or you want to silence the warning, you can set the `BINARYBUILDER_RUNNER` environment variable to `privileged`.  Unfortunately, this involves running `sudo` every time you launch into a BinaryBuilder session, but on the other hand, this successfully works around the issue on distributions such as Arch linux.