_context/doc/modules: add GOPRIVATE to managing-dependencies

Fixes golang/go#44240

Change-Id: I86d07db963fb0cebd5e6560422a2500d9b0d5164
Reviewed-on: https://go-review.googlesource.com/c/website/+/348990
Reviewed-by: jiahua wang <wjh180909@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Steve Traut <straut@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/_content/doc/modules/managing-dependencies.md b/_content/doc/modules/managing-dependencies.md
index aaf5fcb..7f38188 100644
--- a/_content/doc/modules/managing-dependencies.md
+++ b/_content/doc/modules/managing-dependencies.md
@@ -519,3 +519,18 @@
     ```
     GOPROXY="https://proxy.example.com|https://proxy2.example.com"
     ```
+
+
+Go modules are frequently developed and distributed on version control servers
+and module proxies that aren’t available on the public internet. You can set the
+`GOPRIVATE` environment variables. You can set the `GOPRIVATE` environment variable
+to configure the `go` command to download and build modules from private sources.
+Then the go command can download and build modules from private sources.
+
+The `GOPRIVATE` or `GONOPROXY` environment variables may be set to lists of glob
+patterns matching module prefixes that are private and should not be requested
+from any proxy. For example:
+
+```
+GOPRIVATE=*.corp.example.com,*.research.example.com
+```