Update Readme about Tideways

**Describe the bug**
Flame Graph does not build with Tideways on PHP8, only with Xhprof

**To Reproduce**
Steps to reproduce the behavior:
1. Install `tideways_xhprof.so` on PHP 8.
2. Run script for profiling.
3. Aggregate details.
4. Go to Flame Graph.
5. See empty Flame Graph.

**Expected behavior**
See full Flame Graph.

**Screenshots**

**Additional context**
When I switched to the Xhprof extension, collect profile details again, and aggregated them.
After I saw the full Flame Graph.
```bash
$ php -dextension=xhprof -v
PHP 8.0.12 (cli) (built: Oct 22 2021 12:34:00) ( NTS )
Copyright (c) The PHP Group
Zend Engine v4.0.12, Copyright (c) Zend Technologies
with Zend OPcache v8.0.12, Copyright (c), by Zend Technologies

$ php -dextension=xhprof -m
[PHP Modules]
apcu
bcmath
calendar
Core
ctype
curl
date
dom
exif
FFI
fileinfo
filter
ftp
gd
gettext
gmp
hash
iconv
imagick
intl
json
libxml
mbstring
mysqli
mysqlnd
newrelic
openssl
pcntl
pcre
PDO
pdo_mysql
Phar
posix
readline
Reflection
session
shmop
SimpleXML
soap
sockets
sodium
SPL
ssh2
standard
sysvmsg
sysvsem
sysvshm
tokenizer
xhprof
xml
xmlreader
xmlwriter
xsl
Zend OPcache
zlib

[Zend Modules]
Zend OPcache
```

Start with the README context in the issue and reproduce the empty Flame Graph using Tideways on PHP 8, then compare it with the working Xhprof setup. Trace the Flame Graph aggregation path and confirm whether the completed work should document Tideways support or restore full graph output; done means the expected behavior is verified.

Written by the indexing model from the issue text.