doc: Add install guide instructions.

docs/guides/install.md

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+# Installing pikl
+
+## From crates.io (recommended)
+
+```sh
+cargo install pikl
+```
+
+This builds the unified `pikl` binary with both TUI and
+GUI frontends. You'll need a working Rust toolchain. If
+you don't have one, [rustup](https://rustup.rs) is the
+way to go.
+
+### TUI only
+
+If you only want the terminal interface and don't want to
+pull in GUI dependencies:
+
+```sh
+cargo install pikl --no-default-features --features tui
+```
+
+## From source
+
+```sh
+git clone https://github.com/maplecool/pikl-menu.git
+cd pikl-menu
+cargo install --path .
+```
+
+This builds and installs the `pikl` binary into your
+cargo bin directory (usually `~/.cargo/bin/`).
+
+## Package managers
+
+We'd like pikl to be available in package managers like
+the AUR and Homebrew, but honestly haven't set that up
+before and aren't sure when we'll get to it. TBD.
+
+If you package pikl for a distro or package manager, open
+an issue and we'll link it here.
+
+## Verify it works
+
+```sh
+echo -e "hello\nworld\ngoodbye" | pikl
+```
+
+You should see a filterable list in insert mode. Type to
+filter, use arrow keys to navigate, Enter to select,
+Escape to quit. The selected item prints to stdout.
+
+pikl starts in insert mode by default (type to filter
+immediately). Press Ctrl+N to switch to normal mode for
+vim-style navigation (j/k, gg, G, Ctrl+D/U). Ctrl+E
+switches back to insert mode. You can also start in
+normal mode with `--start-mode normal`.
+
+Note: Ctrl+I is not the same as Tab. pikl treats these
+as distinct inputs.