docs: fix build instructions (#4123)

docs/app-dev/getting-started.md

@@ -21,10 +21,13 @@ using Tendermint.
 ### Install
 
 The first apps we will work with are written in Go. To install them, you
-need to [install Go](https://golang.org/doc/install) and put
-`$GOPATH/bin` in your `$PATH`; see
-[here](https://github.com/tendermint/tendermint/wiki/Setting-GOPATH) for
-more info.
+need to [install Go](https://golang.org/doc/install), put
+`$GOPATH/bin` in your `$PATH` and enable go modules with these instructions:
+```bash
+echo export GOPATH=\"\$HOME/go\" >> ~/.bash_profile
+echo export PATH=\"\$PATH:\$GOPATH/bin\" >> ~/.bash_profile
+echo export GO111MODULE=on >> ~/.bash_profile
+```
 
 Then run
 

docs/introduction/install.md

@@ -18,7 +18,12 @@ To download pre-built binaries, see the [releases page](https://github.com/tende
 ## From Source
 
 You'll need `go` [installed](https://golang.org/doc/install) and the required
-[environment variables set](https://github.com/tendermint/tendermint/wiki/Setting-GOPATH)
+environment variables set, which can be done with the following commands:
+```bash
+echo export GOPATH=\"\$HOME/go\" >> ~/.bash_profile
+echo export PATH=\"\$PATH:\$GOPATH/bin\" >> ~/.bash_profile
+echo export GO111MODULE=on >> ~/.bash_profile
+```
 
 ### Get Source Code