Documentation: add modules mode note to install documentation

1. Adds a note to the documentation describing how to use 'go get' in
   modules mode
2. Removes the '-u' option which, when 'go get' is used incorrectly
   will make 'go' try to compile Delve with unsupported dependencies
3. Removed obsolete note about Go 1.5 vendor experiment.

Fixes #1988

Documentation/installation/freebsd/install.md

@@ -5,9 +5,11 @@ Please use the following steps to build and install Delve on FreeBSD.
 There are two ways to install on FreeBSD. First is the standard `go get` method:
 
 ```
-go get -u github.com/go-delve/delve/cmd/dlv
+go get github.com/go-delve/delve/cmd/dlv
 ```
 
+Note: if you are using Go in modules mode you must execute this command outside of a module directory or Delve will be added to your project as a dependency.
+
 Alternatively make sure $GOPATH is set (e.g. as `~/.go`) and:
 
 ```
@@ -16,4 +18,3 @@ $ cd $GOPATH/src/github.com/go-delve/delve
 $ gmake install
 ```
 
-Note: If you are using Go 1.5 you must set `GO15VENDOREXPERIMENT=1` before continuing. The `GO15VENDOREXPERIMENT` env var simply opts into the [Go 1.5 Vendor Experiment](https://docs.google.com/document/d/1Bz5-UB7g2uPBdOx-rw5t9MxJwkfpx90cqG9AFL0JAYo/).

Documentation/installation/linux/install.md

@@ -5,9 +5,11 @@ Please use the following steps to build and install Delve on Linux.
 There are two ways to install on Linux. First is the standard `go get` method:
 
 ```
-go get -u github.com/go-delve/delve/cmd/dlv
+go get github.com/go-delve/delve/cmd/dlv
 ```
 
+Note: if you are using Go in modules mode you must execute this command outside of a module directory or Delve will be added to your project as a dependency.
+
 Alternatively make sure $GOPATH is set (e.g. as `~/.go`) and:
 
 ```
@@ -15,5 +17,3 @@ $ git clone https://github.com/go-delve/delve.git $GOPATH/src/github.com/go-delv
 $ cd $GOPATH/src/github.com/go-delve/delve
 $ make install
 ```
-
-Note: If you are using Go 1.5 you must set `GO15VENDOREXPERIMENT=1` before continuing. The `GO15VENDOREXPERIMENT` env var simply opts into the [Go 1.5 Vendor Experiment](https://docs.google.com/document/d/1Bz5-UB7g2uPBdOx-rw5t9MxJwkfpx90cqG9AFL0JAYo/).

Documentation/installation/osx/install.md

@@ -9,9 +9,11 @@ This should be as simple as:
 Now you can install delve using `go get`:
 
 ```
-$ go get -u github.com/go-delve/delve/cmd/dlv
+$ go get github.com/go-delve/delve/cmd/dlv
 ```
 
+Note: if you are using Go in modules mode you must execute this command outside of a module directory or Delve will be added to your project as a dependency.
+
 With this method you will not be able to use delve's native backend, *but you don't need it anyway*: the native backend on macOS [has known problems](https://github.com/go-delve/delve/issues/1112) on recent issues of the OS and is not currently maintained.
 
 If you didn't enable Developer Mode using Xcode you will be asked to authorize the debugger every time you use it. To enable Developer Mode and only have to authorize once per session use:

Documentation/installation/windows/install.md

@@ -3,9 +3,9 @@
 Please use the standard `go get` command to build and install Delve on Windows.
 
 ```
-go get -u github.com/go-delve/delve/cmd/dlv
+go get github.com/go-delve/delve/cmd/dlv
 ```
 
-Also, if not already set, you have to add the %GOPATH%\bin directory to your PATH variable.
+Note: if you are using Go in modules mode you must execute this command outside of a module directory or Delve will be added to your project as a dependency.
 
-Note: If you are using Go 1.5 you must set `GO15VENDOREXPERIMENT=1` before continuing. The `GO15VENDOREXPERIMENT` env var simply opts into the [Go 1.5 Vendor Experiment](https://docs.google.com/document/d/1Bz5-UB7g2uPBdOx-rw5t9MxJwkfpx90cqG9AFL0JAYo/).
+Also, if not already set, you have to add the %GOPATH%\bin directory to your PATH variable.