README.md: don't recommend 'go get'

With the latest version of go, the recommended 'go get' commands fail:

    go: go.mod file not found in current directory or any parent directory.
            'go get' is no longer supported outside a module.
            To build and install a command, use 'go install' with a version,
            like 'go install example.com/cmd@latest'
            For more information, see https://golang.org/doc/go-get-install-deprecation
            or run 'go help get' or 'go help install'.

Replace them with commands that work.

README.md

@@ -219,12 +219,12 @@ information about each of the commands.
     - Arch: [`pam`](https://www.archlinux.org/packages/core/x86_64/pam/)
       package (usually installed by default)
 
-Once all the dependencies are installed, you can get the repository by running:
+Once all the dependencies are installed, clone the repository by running:
 ```shell
-go get -d github.com/google/fscrypt/...
+git clone https://github.com/google/fscrypt
 ```
-Running `make` in `$GOPATH/src/github.com/google/fscrypt` builds the binary
-(`fscrypt`) and PAM module (`pam_fscrypt.so`) in the `bin/` directory.
+Running `make` builds the binary (`fscrypt`) and PAM module (`pam_fscrypt.so`)
+in the `bin/` directory.
 
 Running `sudo make install` installs `fscrypt` into `/usr/local/bin`,
 `pam_fscrypt.so` into `/usr/local/lib/security`, and `pam_fscrypt/config` into
@@ -245,7 +245,7 @@ binary, or `make install-pam` to only install the PAM files.
 Alternatively, if you only want to install the `fscrypt` binary to
 `$GOPATH/bin`, simply run:
 ```shell
-go get github.com/google/fscrypt/cmd/fscrypt
+go install github.com/google/fscrypt/cmd/fscrypt@latest
 ```
 
 See the `Makefile` for instructions on how to further customize the build.