[clangd] Update documentation page with new features, instructions

[malaperle]

Summary

• Some features were implemented so mark them as such.
• Add installation instructions (LLVM debian packages)

Signed-off-by: Marc-Andre Laperle <[e-mail]>

[jkorous-apple]

LGTM

clang-tools-extra/trunk/docs/clangd.rst

 :program:`Clangd` is not meant to be used by C/C++ developers directly but
 rather from a client implementing the protocol. A client would be typically
 implemented in an IDE or an editor.
 
 At the moment, `Visual Studio Code <https://code.visualstudio.com/>`_ is mainly
 used in order to test :program:`Clangd` but more clients are likely to make
 use of :program:`Clangd` in the future as it matures and becomes a production
 quality tool. If you are interested in trying :program:`Clangd` in combination
-with Visual Studio Code, you can start by `building Clangd`_, then open Visual
-Studio Code in the clangd-vscode folder and launch the extension.
+with Visual Studio Code, you can start by `installing Clangd`_ or
+`building Clangd`_, then open Visual Studio Code in the clangd-vscode folder and
+launch the extension.
+
+Installing Clangd
+==================
+
+Packages are available for debian-based distributions, see the `LLVM packages
+page <http://apt.llvm.org/>`_. :program:`Clangd` is included in the
+`clang-tools` package.
+However, it is a good idea to check your distribution's packaging system first
+as it might already be available.
+
+Otherwise, you can install :program:`Clangd` by `building Clangd`_ first.
 
 Building Clangd
 ==================
 
 You can follow the instructions for `building Clang
 <https://clang.llvm.org/get_started.html>`_ but "extra Clang tools" is **not**
 optional.
 
 Current Status
 ==================
 
 Many features could be implemented in :program:`Clangd`.
 Here is a list of features that could be useful with the status of whether or
 not they are already implemented in :program:`Clangd` and specified in the
 Language Server Protocol. Note that for some of the features, it is not clear
 whether or not they should be part of the Language Server Protocol, so those
-features might be eventually developed outside :program:`Clangd`.
+features might be eventually developed outside :program:`Clangd` or as an
+extension to the protocol.
 
 +-------------------------------------+------------+----------+
 | C/C++ Editor feature                |  LSP       |  Clangd  |
 +=====================================+============+==========+
 | Formatting                          | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
 | Completion                          | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
 | Diagnostics                         | Yes        |   Yes    |
 +-------------------------------------+------------+----------+ 
 | Fix-its                             | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
 | Go to Definition                    | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
-| Source hover                        | Yes        |   No     |
+| Signature Help                      | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
-| Signature Help                      | Yes        |   No     |
+| Document Highlights                 | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
-| Find References                     | Yes        |   No     |
+| Rename                              | Yes        |   Yes    |
 +-------------------------------------+------------+----------+
-| Document Highlights                 | Yes        |   No     |
+| Source hover                        | Yes        |   No     |
 +-------------------------------------+------------+----------+
-| Rename                              | Yes        |   No     |
+| Find References                     | Yes        |   No     |
 +-------------------------------------+------------+----------+
 | Code Lens                           | Yes        |   No     |
 +-------------------------------------+------------+----------+
+| Document Symbols                    | Yes        |   No     |
++-------------------------------------+------------+----------+
+| Workspace Symbols                   | Yes        |   No     |
++-------------------------------------+------------+----------+
 | Syntax and Semantic Coloring        | No         |   No     |
 +-------------------------------------+------------+----------+
 | Code folding                        | No         |   No     |
 +-------------------------------------+------------+----------+
 | Call hierarchy                      | No         |   No     |
 +-------------------------------------+------------+----------+
 | Type hierarchy                      | No         |   No     |
 +-------------------------------------+------------+----------+