title: how to add include path for language servers in coc-settings.json file?
body: <p>I am using neovim plugin <a href="https://github.com/neoclide/coc.nvim" rel="nofollow noreferrer">"coc-nvim"</a>, as the IntelliSense tool for C/C++. as I know this plugin is actually a client of language servers like ccls, clangd and cquery. I have all of these language servers installed on my system and configured them to work with coc-nvim. so I can get autocompletion and list of general headers(e.g. stdio and ...) in my c/c++ files. but since I work in ros platform I want to add the headers of the ros(/opt/ros/kinetic/include/**) to my default include path of language servers. with out this option, at the beginning of each project I must add a dummy ros node and compile with <code>catkin_make -DCMAKE_EXPORT_COMPILE_COMMANDS=YES</code> to generate the compile_commands.json file to get access to ros headers.</p>
  
  <p>the settings of coc.nvim is alongside the init.vim file, in a file called "coc-settings.json" and the contents of my coc setting file is as below:</p>
  
  <pre><code>{
  
  "languageserver": {
      "clangd": {
        "command": "clangd",
        "rootPatterns": ["compile_flags.txt", "compile_commands.json"],
        "filetypes": ["c", "cpp", "objc", "objcpp"]
      },
      "cquery": {
        "command": "/home/amirrezasadeghi/ManBuildLibs/cquery/build/cquery",
        "args": ["--log-file=/tmp/cq.log"],
        "filetypes": ["c", "cpp"],
        "rootPatterns": ["compile_flags.txt", "compile_commands.json", ".git/", ".hg/"],
        "initializationOptions": {
          "cacheDirectory": "/tmp/cquery"
        }
      },
      "ccls": {
        "command": "ccls",
        "filetypes": ["c", "cpp", "objc", "objcpp"],
        "rootPatterns": [".ccls", "compile_commands.json", ".git/", ".hg/"],
        "initializationOptions": {
           "cache": {
             "directory": "/tmp/ccls"
           }
         }
      }
    },
    "clangd.semanticHighlighting": true
  }
  </code></pre>
  
  <p>So my major question is how to add ros include path to these settings. I checked the <a href="https://github.com/neoclide/coc.nvim/wiki" rel="nofollow noreferrer">wiki page of coc.nvim</a> but can't find any option for user include path. also, I checked the option of clangd and tried to add, 
  <code>"args":["-I/opt/ros/kinetic/include/"]</code>,
  to the corresponding block but it did not work. also, I added <a href="https://github.com/cquery-project/cquery/wiki/.cquery" rel="nofollow noreferrer">.cquery</a> file, but still no effect on the list of the proposed headers.</p>
  
  <p>sorry for long story, and thanks for your time.</p>
  
  <p>Edit: it would be better to use the clangd LSP rather than ccls. because it is faster and has better documentation. thanks!</p>
  
  <p>Edit(12 April 2020): the best solution which I found is using ccls language server by using the <code>extraArgs</code> which is provided by ccls. so if you add <code>"clang"</code> field to ccls configs we can have ros headers in autocompletion.</p>
  
  <pre><code>"initializationOptions": {                                                                                        
                                      "cache": {                                                                                                
                                              "directory": "/tmp/ccls"                
                                      },                                              
                                      "clang": {                                      
                                              "extraArgs": [                          
                                                      "-I",                           
                                                      "/opt/ros/kinetic/include/"     
                                              ]                                       
                                      }                                               
                              }
  </code></pre>
  
  <p>but as I mentioned I would like to use clangd if possible. I checked everywhere but no answer. can anyone help me? (please!).</p>