add documentation on Terraform's available build options

BUILDING.md

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+# Building from Source
+
+Pre-built binaries are available for download for a variety of supported platforms through the [HashiCorp Releases website](https://releases.hashicorp.com/terraform/). 
+
+However, if you'd like to build Terraform yourself, you can do so using the Go build toolchain with required options.
+
+## Prerequisites
+
+1. Ensure you've installed the Go language version specified in [`.go-version`](https://github.com/hashicorp/terraform/blob/main/.go-version).
+2. Clone this repository to a location of your choice.
+
+## Terraform Build Options
+
+Terraform accepts certain options passed using `ldflags` at build time which control the behavior of the resulting binary.
+
+### `github.com/hashicorp/terraform/version.useDevelopmentVersion`
+
+Terraform will include a `-dev` flag when reporting its own version (ex: 1.5.0-dev) unless this option is set to `no`:
+
+```
+go build -ldflags "-w -s -X 'github.com/hashicorp/terraform/version.useDevelopmentVersion=no'" -o bin/ .
+```
+
+### `main.experimentsAllowed`
+
+Experimental features of Terraform will be disabled unless this option is set to `yes`:
+
+```
+go build -ldflags "-w -s -X 'main.experimentsAllowed=yes'" -o bin/ .
+```
+
+## Go Options
+
+The Terraform release process uses the Go toolchain defaults for the given operating system and processor architecture.
+
+