[PATCH 2/3] erofs-utils: fix typos and enhance installation guide

[Saksham]

This patch addresses several issues in the README and docs/INSTALL.md
to improve the overall documentation quality and provide a better
experience for new users and developers.

First, multiple instances of "plusters" were found in the README file.
These were typos for "pclusters" (physical clusters), which is a key
concept in EROFS for block-level compression and data management.
Correcting these ensures technical accuracy and avoids confusion
for users trying to understand the filesystem's behavior, especially
regarding the "big pcluster" feature introduced in Linux 5.13.

Specifically:
- Fixed "big plusters" to "big pclusters" in the section describing
  high-compression image generation.
- Fixed "4k plusters" to "4k pclusters" in the compression hints
  example section.

Second, the installation documentation in docs/INSTALL.md was updated
to provide a more streamlined onboarding process. A "Quick Start"
section was added at the top, listing all common prerequisites for
Debian-based systems (Ubuntu, etc.). This allows users to quickly
get all necessary libraries (lz4, xz, uuid, fuse, etc.) and build
the project with a single set of commands.

Third, a new section was added to docs/INSTALL.md regarding
multithreading support. While multithreading is enabled by default
in mkfs.erofs if the compiler and environment support it, it was
not explicitly documented in the INSTALL guide. The new section
explains how to explicitly enable it with --enable-multithreading
or disable it with --disable-multithreading, providing users with
more control over their build configuration.

These changes ensure that the documentation remains up-to-date
with the latest features of erofs-utils and provides clear
instructions for both new and experienced users.
---
 README          |  4 ++--
 docs/INSTALL.md | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/README b/README
index 1ca376f..6f9e761 100644
--- a/README
+++ b/README
@@ -122,7 +122,7 @@ images.  Users may prefer smaller images for archiving purposes, even if
 random performance is compromised with those configurations, and even
 worse when using 4KiB blocks.
 
-In order to fulfill users' needs, big plusters has been introduced
+In order to fulfill users' needs, big pclusters has been introduced
 since Linux 5.13, in which each physical clusters will be more than one
 blocks.
 
@@ -159,7 +159,7 @@ write a compress-hints file like below:
 and specify with `--compress-hints=` so that ".so" files will use
 "lz4hc,12" compression with 4k pclusters, ".txt" files will use
 "lzma,9" compression with 32k pclusters, files  under "/sbin" will use
-the default "lzma" compression with 4k plusters and other files will
+the default "lzma" compression with 4k pclusters and other files will
 use "lzma" compression with 16k pclusters.
 
 Note that the largest pcluster size should be specified with the "-C"
diff --git a/docs/INSTALL.md b/docs/INSTALL.md
index 2e818da..d96b15c 100644
--- a/docs/INSTALL.md
+++ b/docs/INSTALL.md
@@ -4,6 +4,26 @@ source.
 See the [README](../README) file in the top level directory about
 the brief overview of erofs-utils.
 
+## Quick Start
+
+For those who want a quick build, ensure that the following prerequisites are
+installed (on Debian/Ubuntu):
+
+``` sh
+$ sudo apt-get install autoconf automake libtool pkg-config uuid-dev \
+                       liblz4-dev liblzma-dev libfuse-dev zlib1g-dev \
+                       libselinux1-dev libzstd-dev
+```
+
+Then, run the following commands to build and install:
+
+``` sh
+$ ./autogen.sh
+$ ./configure
+$ make
+# make install
+```
+
 ## Dependencies & build
 
 LZ4 1.9.3+ for LZ4(HC) enabled [^1].
@@ -45,6 +65,18 @@ $ make
 Additionally, you could specify liblzma target paths with
 `--with-liblzma-incdir` and `--with-liblzma-libdir` manually.
 
+## How to build with multithreading
+
+To enable multithreading support for mkfs.erofs, use the following:
+
+``` sh
+$ ./configure --enable-multithreading
+$ make
+```
+
+Note that multithreading is enabled by default if the compiler supports it.
+To disable it explicitly, use `--disable-multithreading`.
+
 ## How to build erofsfuse
 
 It's disabled by default as an experimental feature for now due
-- 
2.53.0