Add warning when 'conceallevel' not set correctly

And add another note in the README. Closes #286.

README.md

@@ -379,6 +379,7 @@ This is a complete list of all of the options that can be passed to `require("ob
   open_notes_in = "current",
 
   -- Optional, configure additional syntax highlighting / extmarks.
+  -- This requires you have `conceallevel` set to 1 or 2. See `:help conceallevel` for more details.
   ui = {
     enable = true,  -- set to false to disable all additional syntax features
     update_debounce = 200,  -- update delay after a text change (in milliseconds)
@@ -476,7 +477,7 @@ If you use `vim-markdown` you'll probably want to disable its frontmatter syntax
 
 #### Concealing characters
 
-If you wish to use the formatting concealment features, you will need to have `conceallevel` set to a value that allows it, for example:
+If you wish to use the formatting concealment features, you will need to have `conceallevel` set to a value that allows it (either `1` or `2`), for example:
 `set conceallevel=1` in viml or `vim.opt.conceallevel = 1` in a lua config.
 
 #### Note naming and location

lua/obsidian/ui.lua

@@ -567,6 +567,16 @@ M.setup = function(ui_opts)
     return
   end
 
+  local conceallevel = vim.opt_local.conceallevel:get()
+  if conceallevel < 1 or conceallevel > 2 then
+    log.warn(
+      "Obsidian additional syntax features require 'conceallevel' to be set to 1 or 2, "
+        .. "but you have 'conceallevel' set to '%s'.\n"
+        .. "See https://github.com/epwalsh/obsidian.nvim/issues/286 for more details.",
+      conceallevel
+    )
+  end
+
   local group = vim.api.nvim_create_augroup("obsidian_ui", { clear = true })
 
   install_hl_groups(ui_opts)