Installation may fail on Linux if libcurl is not installed

[nrejac]

Installing via install.packages("ridigbio") may fail in Linux is libcurl is not installed. Document how to fix this.

Console log messages:

* installing *source* package ‘curl’ ...
** package ‘curl’ successfully unpacked and MD5 sums checked
** using staged installation
Package libcurl was not found in the pkg-config search path.
Perhaps you should add the directory containing `libcurl.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libcurl' found
Package libcurl was not found in the pkg-config search path.
Perhaps you should add the directory containing `libcurl.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libcurl' found
Using PKG_CFLAGS=
Using PKG_LIBS=-lcurl
------------------------- ANTICONF ERROR ---------------------------
Configuration failed because libcurl was not found. Try installing:
 * deb: libcurl4-openssl-dev (Debian, Ubuntu, etc)
 * rpm: libcurl-devel (Fedora, CentOS, RHEL)
 * csw: libcurl_dev (Solaris)
If libcurl is already installed, check that 'pkg-config' is in your
PATH and PKG_CONFIG_PATH contains a libcurl.pc file. If pkg-config
is unavailable you can set INCLUDE_DIR and LIB_DIR manually via:
R CMD INSTALL --configure-vars='INCLUDE_DIR=... LIB_DIR=...'
--------------------------------------------------------------------
ERROR: configuration failed for package ‘curl’
* removing ‘/home/nrejack/R/x86_64-pc-linux-gnu-library/4.0/curl’
Warning in install.packages :
  installation of package ‘curl’ had non-zero exit status

and later:

ERROR: dependency ‘curl’ is not available for package ‘httr’
* removing ‘/home/nrejack/R/x86_64-pc-linux-gnu-library/4.0/httr’
Warning in install.packages :
  installation of package ‘httr’ had non-zero exit status
ERROR: dependency ‘httr’ is not available for package ‘ridigbio’
* removing ‘/home/nrejack/R/x86_64-pc-linux-gnu-library/4.0/ridigbio’
Warning in install.packages :
  installation of package ‘ridigbio’ had non-zero exit status