Mention ccls and clangd in README.md

[MaskRay]

Whether they work better in lsp-mode is a QoI issue that is unrelated to this PR.

I have checked that both ccls and clangd (built from llvm+clang+extra trunk) work: textDocument/definition.

[MaskRay]

It's true that I want to promote ccls

[joaotavora]

So ccls is now the default. I tested it and it works decently well without any extra configuration. It's a pity that it still has this abandoned serious cquery bug though :-(

[MaskRay]

Implemented index.onChange: true in git HEAD. I didn't implement this when I rewrote the pipeline.

https://github.com/MaskRay/ccls/blob/master/src/config.h#L204

This has very bad performance impact and currently the cache file will be saved every time a change is made.

[joaotavora]

Implemented index.onChange: true in git HEAD. I didn't implement this when I rewrote the pipeline.

I don't have time to test now, but if I remember correctly (and if you read the bug I linked to) this isn't solved by index.onChange: true

[MaskRay]

I have just added support for empty cacheDirectory (default is ".ccls-cache")

{
  "cacheDirectory": ""
  "index": {
    "onChange": true
  }
}

This is on-the-fly indexing without saving cache files.