docs: update install docs to no longer use go get

Fixes lightningnetwork#4444.

Author: Roasbeef

docs/INSTALL.md

@@ -104,10 +104,24 @@
 With the preliminary steps completed, to install `lnd`, `lncli`, and all
 related dependencies run the following commands:
 ```
-go get -d github.com/lightningnetwork/lnd
-cd $GOPATH/src/github.com/lightningnetwork/lnd
-make && make install
+git clone https://github.com/lightningnetwork/lnd
+cd lnd
+make install
+```
+
+The command above will install the current _master_ branch of `lnd`. If you
+wish to install a tagged release of `lnd` (as the master branch can at times be
+unstable), then [visit then release page to locate the latest
+release](https://github.com/lightningnetwork/lnd/releases). Assuming the name
+of the release is `v0.x.x`, then you can compile this release from source with
+a small modification to the above command: 
 ```
+git clone https://github.com/lightningnetwork/lnd
+cd lnd
+git checkout v0.x.x
+make install
+```
+
 
 **NOTE**: Our instructions still use the `$GOPATH` directory from prior
 versions of Go, but with Go 1.12, it's now possible for `lnd` to live