Create docs/ folder with initial content

Add more documentation on the following topics:
* Running `curl-impersonate` from the command line, including changing
the HTTP headers when needed.
* Using `libcurl-impersonate` in JS and PHP scripts.

docs/03_LIBCURL_IMPERSONATE_PHP.md

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+# Using libcurl-impersonate in PHP scripts
+
+It is possible to use libcurl-impersonate in PHP scripts instead of the original libcurl. PHP loads libcurl dynamically during runtime, which means that a different set of steps needs to be taken.
+
+## On Linux
+
+First, patch libcurl-impersonate and change its SONAME:
+```bash
+patchelf --set-soname libcurl.so.4 /path/to/libcurl-impersonate-chrome.so
+```
+
+Then replace at runtime with:
+```bash
+LD_PRELOAD=/path/to/libcurl-impersonate-chrome.so CURL_IMPERSONATE=chrome101 php -r 'print_r(curl_version());'
+```
+
+If successful you should see:
+```
+[ssl_version] => BoringSSL
+```
+(or NSS if the Firefox version is used)
+
+## On macOS
+
+On Mac, first rename `libcurl-impersonate-chrome.dylib` to `libcurl.4.dylib` and place in some directory, say `/usr/local/lib`. Then run php with the `DYLD_LIBRARY_PATH` env var pointing to that directory, for example:
+```
+DYLD_LIBRARY_PATH=/usr/local/lib php -r 'print_r(curl_version());'
+```
+
+If successful you should see:
+```
+[ssl_version] => BoringSSL
+```