Auto merge of #2396 - dpzmick:patch-1, r=micbou

Update README.md

# PR Prelude

Thank you for working on YCM! :)

**Please complete these steps and check these boxes (by putting an `x` inside
the brackets) _before_ filing your PR:**

- [x] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [x] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [x] I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.
- [x] **I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.**

[cont]: https://github.com/Valloric/YouCompleteMe/blob/master/CONTRIBUTING.md
[code]: https://github.com/Valloric/YouCompleteMe/blob/master/CODE_OF_CONDUCT.md

# Why this change is necessary and useful
ycmd has this support: https://github.com/Valloric/ycmd/pull/628

no tests needed, only a change to docs

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README.md

@@ -732,6 +732,7 @@ Quick Feature Summary
 * Go to definition (`GoTo`, `GoToDefinition`, and `GoToDeclaration` are
   identical)
 * Management of `racer` server instance
+* View documentation comments for identifiers (`GetDoc`)
 
 User Guide
 ----------
@@ -1363,7 +1364,7 @@ under the cursor. Depending on the file type, this includes things like:
 * etc.
 
 Supported in filetypes: `c, cpp, objc, objcpp, cs, python, typescript,
-javascript`
+javascript, rust`
 
 #### The `GetDocImprecise` subcommand
 

doc/youcompleteme.txt

@@ -966,6 +966,7 @@ Rust ~
 - Go to definition (|GoTo|, |GoToDefinition|, and |GoToDeclaration| are
   identical)
 - Management of 'racer' server instance
+- View documentation comments for identifiers (|GetDoc|)
 
 ===============================================================================
                                                      *youcompleteme-user-guide*
@@ -1639,7 +1640,7 @@ under the cursor. Depending on the file type, this includes things like:
 - etc.
 
 Supported in filetypes: 'c, cpp, objc, objcpp, cs, python, typescript,
-javascript'
+javascript, rust'
 
 -------------------------------------------------------------------------------
 The *GetDocImprecise* subcommand