docs: convert BACKERS.md to backers.txt

There is no reason for this file to be in project root, which is crowded
as is. This also fits nicely part of the ongoing work towards gathering
as much of the documentation as possible into one place.

test/functional/editor/completion_spec.lua

@@ -895,18 +895,18 @@ describe('completion', function()
 
   it("'ignorecase' 'infercase' CTRL-X CTRL-N #6451", function()
     feed_command('set ignorecase infercase')
-    feed_command('edit BACKERS.md')
+    feed_command('edit runtime/doc/backers.txt')
     feed('oX<C-X><C-N>')
-    screen:expect([[
-      # Bountysource Backers                                      |
+    screen:expect{grid=[[
+      *backers.txt*          Nvim                                 |
       Xnull^                                                       |
       {2:Xnull          }{6: }                                            |
-      {1:Xoxomoon       }{6: }ryone who backed our [Bountysource fundraise|
-      {1:Xu             }{6: }ountysource.com/teams/neovim/fundraiser)!   |
+      {1:Xoxomoon       }{6: }                                            |
+      {1:Xu             }{6: }     NVIM REFERENCE MANUAL                  |
       {1:Xpayn          }{2: }                                            |
-      {1:Xinity         }{2: }d URL in BACKERS.md.                        |
+      {1:Xinity         }{2: }                                            |
       {3:-- Keyword Local completion (^N^P) }{4:match 1 of 7}             |
-    ]])
+    ]]}
   end)
 
   it('CompleteChanged autocommand', function()