docs: documentation for `buildEnv` (#592)

* Doc for func.yaml's buildEnv

* Update docs/guides/func_yaml.md

Co-authored-by: Zbynek Roubalik <726523+zroubalik@users.noreply.github.com>

Co-authored-by: Zbynek Roubalik <726523+zroubalik@users.noreply.github.com>

docs/guides/func_yaml.md

@@ -28,6 +28,26 @@ field will contain all of the available builders for a given runtime. Although
 it's typically unnecessary to modify the `builder` field, using values from
 `builders` is OK.
 
+### `buildEnvs`
+This field allows you to set environment variables available to the builder/buildpack that builds the function. This environment variable is NOT set at runtime, use [envs](#envs) instead
+1. Environment variable can be set directly from a value
+2. Environment variable can be set from a local environment value. Eg. `'{{ env:LOCAL_ENV_VALUE }}'`, for more details see [Local Environment Variables section](#local-environment-variables).
+
+```yaml
+buildEnvs:
+- name: EXAMPLE1                            # (1) env variable directly from a value
+  value: value
+- name: EXAMPLE2                            # (2) env variable from a local environment value
+  value: '{{ env:LOCAL_ENV_VALUE }}'
+```
+
+For example, the below `func.yaml` snippet modifies the default Golang buildpack to build source code with 1.15 compiler version. Refer to respective buildpack documentation to know more about environment variables that modify behavior of the `func build`.
+```yaml
+buildEnvs:
+- name: BP_GO_VERSION
+  value: '1.15'
+```
+
 ### `envs`
 
 The `envs` field allows you to set environment variables that will be