ARROW-6182: [R] Add note to README about r-arrow conda installation

cc @xhochy

Closes #5142 from nealrichardson/r-conda-install and squashes the following commits:

71a76d0 <Neal Richardson> Add instructions on installing from conda to the R README

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

r/README.Rmd

@@ -30,13 +30,13 @@ Install the latest release of `arrow` from CRAN with
 install.packages("arrow")
 ```
 
-> Warning: the macOS binary packages on CRAN for 0.14.1 do not work as described
-> below. To install, you'll first need to use Homebrew to get the Arrow C++
-> library (`brew install apache-arrow`), and then from R you can
-> `install.packages("arrow", type = "source")`. We hope to have this resolved
-> in the next release.
+Conda users on Linux and macOS can install `arrow` from conda-forge with
 
-On macOS and Windows, installing a binary package from CRAN will handle Arrow's C++ dependencies for you. On Linux, you'll need to first install the C++ library. See the [Arrow project installation page](https://arrow.apache.org/install/) to find pre-compiled binary packages for some common Linux distributions, including Debian, Ubuntu, and CentOS. You'll need to install `libparquet-dev` on Debian and Ubuntu, or `parquet-devel` on CentOS. This will also automatically install the Arrow C++ library as a dependency. Other Linux distributions must install the C++ library from source.
+```
+conda install -c conda-forge r-arrow
+```
+
+On macOS and Windows, installing a binary package from CRAN will handle Arrow's C++ dependencies for you. On Linux, unless you use `conda` you'll need to first install the C++ library. See the [Arrow project installation page](https://arrow.apache.org/install/) to find pre-compiled binary packages for some common Linux distributions, including Debian, Ubuntu, and CentOS. You'll need to install `libparquet-dev` on Debian and Ubuntu, or `parquet-devel` on CentOS. This will also automatically install the Arrow C++ library as a dependency. Other Linux distributions must install the C++ library from source.
 
 If you install the `arrow` package from source and the C++ library is not found, the R package functions will notify you that Arrow is not available. Call
 

r/README.md

@@ -32,21 +32,20 @@ Install the latest release of `arrow` from CRAN with
 install.packages("arrow")
 ```
 
-> Warning: the macOS binary packages on CRAN for 0.14.1 do not work as
-> described below. To install, you’ll first need to use Homebrew to get
-> the Arrow C++ library (`brew install apache-arrow`), and then from R
-> you can `install.packages("arrow", type = "source")`. We hope to have
-> this resolved in the next release.
+Conda users on Linux and macOS can install `arrow` from conda-forge with
+
+    conda install -c conda-forge r-arrow
 
 On macOS and Windows, installing a binary package from CRAN will handle
-Arrow’s C++ dependencies for you. On Linux, you’ll need to first install
-the C++ library. See the [Arrow project installation
-page](https://arrow.apache.org/install/) to find pre-compiled binary
-packages for some common Linux distributions, including Debian, Ubuntu,
-and CentOS. You’ll need to install `libparquet-dev` on Debian and
-Ubuntu, or `parquet-devel` on CentOS. This will also automatically
-install the Arrow C++ library as a dependency. Other Linux distributions
-must install the C++ library from source.
+Arrow’s C++ dependencies for you. On Linux, unless you use `conda`
+you’ll need to first install the C++ library. See the [Arrow project
+installation page](https://arrow.apache.org/install/) to find
+pre-compiled binary packages for some common Linux distributions,
+including Debian, Ubuntu, and CentOS. You’ll need to install
+`libparquet-dev` on Debian and Ubuntu, or `parquet-devel` on CentOS.
+This will also automatically install the Arrow C++ library as a
+dependency. Other Linux distributions must install the C++ library from
+source.
 
 If you install the `arrow` package from source and the C++ library is
 not found, the R package functions will notify you that Arrow is not