docs: explain building from source in README

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
   main:
     runs-on: ubuntu-latest
     env:
-      CANDLE_BUILD: true
+      NATIVE_BUILD: true
     strategy:
       fail-fast: false
       matrix:

README.md

@@ -23,6 +23,25 @@ Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_do
 and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
 be found at <https://hexdocs.pm/candlex>.
 
+## Usage
+
+#### `NATIVE_BUILD`
+
+Defaults to `false`. If `true` the native binary is built locally, which may be useful
+if no precompiled binary is available for your target environment. Once set, you
+must run `mix deps.clean candlex --build` explicitly to force to recompile.
+Building has a number of dependencies, see *Building from source* below.
+
+## Building from source
+
+To build the native binary locally you need to set `NATIVE_BUILD=true`.
+Keep in mind that the compilation usually takes time.
+
+You will need the following installed in your system for the compilation:
+
+  * [Git](https://git-scm.com) for fetching candle-core source
+  * [Rust](https://www.rust-lang.org) with cargo to compile rustler NIFs
+
 ## Releasing
 
 To publish a new version of this package:

lib/candlex/native.ex

@@ -10,7 +10,7 @@ defmodule Candlex.Native do
     otp_app: :candlex,
     features: Application.compile_env(:candlex, :crate_features, []),
     base_url: "#{source_url}/releases/download/v#{version}",
-    force_build: System.get_env("CANDLE_BUILD") in ["1", "true"],
+    force_build: System.get_env("NATIVE_BUILD") in ["1", "true"],
     mode: mode,
     version: version,
     nif_versions: ["2.16"],