Fix prompt exporting escaped characters, opt into PS1 mod.

go/README.md

 # Chromium Infra Go Area
 
 [TOC]
 
 
 ## Get the code
 
 The steps for getting the code are:
 
  1. [Install depot_tools](https://www.chromium.org/developers/how-tos/install-depot-tools)
@@ skipping 78 matching lines @@
 eval `./env.py`
 go install github.com/luci/luci-go/tools/cmd/...
 ./bin/cproto --help  # infra/go/bin is where executables are installed
 cproto --help        # infra/go/bin is also in $PATH
 ```
 
 Alternatively `go/env.py` can be used as a wrapping command that sets up an
 environment and invokes some other process. It is particularly useful on
 Windows.
 
+If the `INFRA_PROMPT_TAG` environment variable is exported while running
+`go/env.py`, the new environment will include a modified `PS1` prompt containing
+the `INFRA_PROMPT_TAG` value to indicate that the modified environment is being
+used. By default, this value is "[cr go] ", but it can be changed by exporting
+a different value or disabled by exporting an empty value.
 
 ## Dependency management
 
 All third party code needed to build `infra/go` is installed into
 `infra/go/.vendor` via `deps.py` script that is invoked as part of the bootstrap
 process.
 
 There are two files that control what code to fetch:
 
    * `deps.yaml` specifies what packages `infra/go` code depends on directly and
@@ skipping 64 matching lines @@
 Some Golang related packages are already on `*.googlesource.com` (though it may
 be non obvious at the first glance). For example all `golang.org/x/*` ones are
 actually served from `https://go.googlesource.com/`.
 
 `deps.py` will warn you if it sees a package being referenced from
 a source-of-truth repo, and not a mirror.
 
 If you are positive that a mirror is needed, file
 [Infra-Git](https://bugs.chromium.org/p/chromium/issues/entry?template=Infra-Git)
 ticket specifying what repository you need to be mirrored.