add BUILDING.md documentation

tfclient/BUILDING.md

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+tfclient has a more involved build process than the rest of trifid due to the fact that it embeds the distribution nebula and nebula-cert binaries.
+
+## Clearing the Nebula binary cache
+
+Compiling tfclient may take longer than usual every once in a while because the build system downloads the latest version of nebula.
+After this has been done once, the generated code will be cached in the target directory. If you want to redownload it (e.g. if Nebula is updated), you can:
+
+- run `cargo clean`, or
+- delete `target/debug/build/tfclient-[HASH]/out/noredownload`, or
+- set the environment variable `TFBUILD_FORCE_REDOWNLOAD=1`
+
+The 3rd option is the best, as it will not signifigantly increase compile times *or* need you to find the build hash.
+
+## Set a GitHub API key
+
+Since the build process involves making calls to the GitHub API, you may get ratelimited.
+To provide a GitHub API token to increase your ratelimit, set `GH_API_KEY` to your GitHub API key.
+
+## Unsupported architecture :(
+
+If you're compiling on (or for) an architecture that tfclient currently does not support, you will get the following message:
+
+```text
+[*] Compiling for target: SOME-TARGET-TRIPLET-HERE
+This architecture is not supported yet :(
+Nebula has a limited set of architectures it is able to function on.
+tfclient can only be compiled on these architectures.
+See https://github.com/slackhq/nebula/releases for a list of supported architectures.
+Is your system supported by Nebula? Shoot a message to the mailing list. Include the target triplet (above) in your response, as well as a link to the functioning Nebula binary. We will happily add your machine to the list!         
+```
+
+The `tfclient` build system attempts to check if your architecture is unsupported. It does this by checking your target triplet.
+If this is a false alarm, and a build of nebula is available and fully functional on your machine, please shoot a message to the mailing list with your target triplet and a link to which build of nebula works on your machine. We will update the build script and update the repo!