Adds workspace command (#61)

* Adds workspace command

* Update 0000-workspaces-command.md

* Update 0000-workspaces-command.md

Author: arcanis

accepted/0000-workspaces-command.md

@@ -0,0 +1,39 @@
+- Start Date: 2017-05-04
+- RFC PR:
+
+# Summary
+
+This document specify a new command, that can be used to execute subcommands inside a project workspaces.
+
+# Motivation
+
+With the addition of the Workspace feature, it will become handy to be able to execute commands inside other workspaces than the one in the current directory.
+
+# Detailed design
+
+This RFC suggests to add the following commands:
+
+## `yarn exec <binary-name> ...`
+
+Execute a shell command inside the same environment than the one used when running scripts. For example, running `yarn exec env` will print something similar to this:
+
+```
+PWD=/path/to/project
+npm_config_user_agent=yarn/0.23.4 npm/? node/v7.10.0 darwin x64
+npm_node_execpath=/usr/bin/node
+...
+```
+
+## `yarn workspace <workspace-name> <command-name> ...`
+
+This command will execute the specified sub-command inside the workspace that is being referenced by `<workspace-name>`.
+
+Recursion aside, it's essentially an alias for:
+
+```
+$> (cd $(yarn workspace <workspace-name> exec pwd) && yarn <command-name> ...)
+```
+
+# Drawbacks
+
+- We will still have the issue of requiring the `--` separator to forward any command line option (`yarn workspace test install -- --production`). It's an important issue, something we really should tackle sooner than later.