faq: add basic FAQ

FAQ.md

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+Frequently Asked Questions (FAQ)
+================================
+
+Why is there a collision error when switching generation?
+---------------------------------------------------------
+
+Home Manager currently installs packages into the user environment,
+precisely as if the packages were installed through
+`nix-env --install`. This means that you will get a collision error if
+your Home Manager configuration attempts to install a package that you
+already have installed manually, that is, packages that shows up when
+you run `nix-env --query`.
+
+For example, imagine you have the `hello` package installed in your
+environment
+
+```console
+$ nix-env --query
+hello-2.10
+```
+
+and your Home Manager configuration contains
+
+    home.packages = [ pkgs.hello ];
+
+Then attempting to switch to this configuration will result in an
+error similar to
+
+```console
+$ home-manager switch
+these derivations will be built:
+  /nix/store/xg69wsnd1rp8xgs9qfsjal017nf0ldhm-home-manager-path.drv
+[…]
+Activating installPackages
+replacing old ‘home-manager-path’
+installing ‘home-manager-path’
+building path(s) ‘/nix/store/b5c0asjz9f06l52l9812w6k39ifr49jj-user-environment’
+Wide character in die at /nix/store/64jc9gd2rkbgdb4yjx3nrgc91bpjj5ky-buildenv.pl line 79.
+collision between ‘/nix/store/fmwa4axzghz11cnln5absh31nbhs9lq1-home-manager-path/bin/hello’ and ‘/nix/store/c2wyl8b9p4afivpcz8jplc9kis8rj36d-hello-2.10/bin/hello’; use ‘nix-env --set-flag priority NUMBER PKGNAME’ to change the priority of one of the conflicting packages
+builder for ‘/nix/store/b37x3s7pzxbasfqhaca5dqbf3pjjw0ip-user-environment.drv’ failed with exit code 2
+error: build of ‘/nix/store/b37x3s7pzxbasfqhaca5dqbf3pjjw0ip-user-environment.drv’ failed
+```
+
+The solution is typically to uninstall the package from the
+environment using `nix-env --uninstall` and reattempt the Home Manager
+generation switch.
+
+Why are the session variables not set?
+--------------------------------------
+
+Home Manager is only able to set session variables automatically if it
+manages your Bash or Z shell configuration. If you don't want to let
+Home Manager manage your shell then you will have to manually source
+the
+
+    ~/.nix-profile/etc/profile.d/hm-session-vars.sh
+
+file in an appropriate way. In Bash and Z shell this can be done by
+adding
+
+```sh
+. "$HOME/.nix-profile/etc/profile.d/hm-session-vars.sh"
+```
+
+to your `.profile` and `.zshrc` files, respectively. The
+`hm-session-vars.sh` file should work in most Bourne-like shells.